Low Serum Naproxen Concentrations Are Associated with Minimal Pain Relief: A Preliminary Study in Women with Dysmenorrhea

Objective Incomplete pain relief after administration of nonsteroidal anti-inflammatory drugs (NSAIDs) is common, but it is unknown whether malabsorption or heightened metabolism contributes to NSAID resistance. To explain the etiology of NSAID resistance, we evaluated naproxen absorption and metabolism in relation to pain relief in a pilot study of women with dysmenorrhea.

During menses, participants completed before and after naproxen ingestion pain assessments. Analgesic effectiveness was calculated as a percent change in pain rating before and after naproxen administration. To evaluate the impact of malabsorption, the correlation between analgesic effectiveness and serum naproxen was analyzed. To identify whether hypermetabolism contributes to NSAID resistance, we also analyzed the metabolite O-desmethylnaproxen.

Serum naproxen and O-desmethylnaproxen concentrations of the dysmenorrheic cohort (N = 23, 126 ± 10 µg/mL, 381 ± 56 ng/mL) and healthy controls (N = 12, 135 ± 8 µg/mL, 355 ± 58 ng/mL) were not significantly different (P > 0.05), suggesting that menstrual pain does not affect drug absorption and metabolism. However, nine dysmenorrhea participants had levels of analgesic effectiveness <30%. Among dysmenorrheic women, analgesic effectiveness was correlated with serum naproxen (r = 0.49, P = 0.019) and O-desmethylnaproxen (r = 0.45, P = 0.032) concentrations. After controlling for other gynecological diagnoses, a multivariate model analysis confirmed that lower serum naproxen concentrations were associated with reduced pain relief (P = 0.038).

Our preliminary findings suggest that poor drug absorption contributes to ineffective pain relief in dysmenorrheic women. Future studies should explore whether malabsorption contributes to NSAID resistance for other pain conditions.

Dysmenorrhea, Menstrual Pain, Nonsteroidal Anti-inflammatory Drug

When nonsteroidal anti-inflammatory drugs (NSAIDs) do not effectively treat common inflammatory pain conditions, clinicians historically may turn to opioids and corticosteroids as second-line options. Thus, it is imperative to identify the mechanisms responsible for NSAID treatment resistance in order to limit the need to resort to drugs with significant adverse effects [1]. NSAID resistance in inflammatory pain conditions may occur for a variety of reasons, ranging from external use–dependent factors, such as medication adherence, dosage, and consumption frequency, to internal factors, such as inadequate absorption or hypermetabolism (for a review, see [2]). Although malabsorption is known to play an important role in treatment resistance for a variety of other conditions such as anticoagulation therapy [3], iron deficiency anemia [4], and HIV [5], its contribution to treatment resistance for pain has been underexplored. One study identified that ∼20% of patients with a mixture of chronic pain conditions do not respond to opioids due to dysfunction of either absorption or metabolism [6]. In arthritis, a condition commonly associated with NSAID resistance, lower steady-state serum or plasma naproxen concentrations were associated with ineffective pain relief [7, 8]. However, these studies did not resolve whether ineffective absorption or metabolism was responsible for treatment resistance. Thus, investigation of absorption and metabolism in a uniform inflammatory pain condition is urgently needed to clarify whether these factors contribute to treatment resistance. Two additional factors could contribute to NSAID resistance: pain-related stress and anatomical factors. For example, it has been shown that stress due to wisdom tooth extraction causes a decrease in serum concentration of ibuprofen enantiomers and a prolongation in the time to peak concentration [9]. Also, the inflammatory state and phenotype of the condition have been hypothesized to affect NSAID effectiveness, but the evidence is typically lacking [2]. According to accepted clinical guidelines, when NSAIDs do not effectively treat period pain, an anatomical cause such as endometriosis, leiomyoma, or adenomyosis should be suspected and potentially evaluated with imaging, followed then by procedural treatments [10]. Given the importance of this clinical decision, studies need to account for anatomical phenotypes as a predictor of NSAID resistance. To explore whether malabsorption and hypermetabolism contribute to ineffective treatment in a common pain condition, we conducted a substudy to profile NSAID absorption and metabolism as part of a larger investigation characterizing the mechanisms of menstrual pain [11, 12]. Although 18% to 30% of women with severe dysmenorrhea receive minimal or no pain relief after taking NSAIDs [13, 14], the role of malabsorption and hypermetabolism has not yet been investigated. We hypothesized that serum concentrations of naproxen would be lower or its metabolite O-desmethylnaproxen (ODN) [15] would be higher in dysmenorrheic women who report minimal pain relief after standardized naproxen administration. To study whether repeated experience of menstrual pain affects naproxen absorption or metabolism, we conducted contrasts between women with and without menstrual pain. Finally, we examined serum concentrations from participants with secondary dysmenorrhea to evaluate the contribution of anatomical factors to NSAID-resistant menstrual pain.

Participant Recruitment We recruited participants with a history of primary or secondary dysmenorrhea and healthy controls (aged 18–45) from physician referral and flyers posted within the community. Participants in this study were enrolled between March 2015 and November 2017. These participants were primarily recruited for a study that investigated the relationship between menstrual pain and abdominal muscle contractions with electromyography (EMG) [12] or uterine contractions in women using magnetic resonance imaging (MRI) [11]. The complete details of recruitment, EMG, and MRI are described in these prior publications. Participant Screening Participants used an 11-point numeric rating scale (NRS; 0 = “no pain at all” to 10 = “worst pain imaginable”) [16] to log the severity of menstrual pain throughout their menstrual cycle for a month with a REDCap [17] Web-based daily diary. To be enrolled as a healthy control, participants were required to rate their menstrual pain (on the worst day during menses) <3 on an NRS and not have any chronic pain. Enrolled dysmenorrheic participants were required to rate their menstrual pain ≥6 on an NRS to ensure a broad range of treatment response. As menstrual pain exists across a continuum, these thresholds were selected to reflect very mild pain (<3) or moderate to severe pain (≥6) [18]. Participants with a history of secondary dysmenorrhea all had a prior surgical diagnosis of endometriosis or imaging confirmation of leiomyoma and ovarian cysts. Participants from all study groups were excluded if they had a history of pelvic or abdominal malignancies, irregular menses (>45 days between menses), a pregnancy within the prior six months, ongoing breastfeeding, an active genitourinary infection in the previous four weeks, a body mass index >40 kg/m2, an unwillingness to stop taking NSAIDs for study visits, an unwillingness to have a withdrawal bleed if on continuous oral contraceptive pills (OCPs), an inability to read/comprehend a consent form in English, or if they had standard MRI contraindications (because of the parent MRI study). Healthy controls could enroll while taking OCPs as long as they did not have a history of menstrual pain. Participants with primary or secondary dysmenorrhea could enroll while taking OCPs if the OCPs did not significantly alleviate their menstrual pain. Study Visit Participants were scheduled for a menses visit during the first 72 hours following menstrual bleeding onset. Participants were instructed to abstain from taking short-acting analgesic medications at least eight hours before the visit, or 12 hours for long-acting analgesics. Upon arrival, participants gave written informed consent. They were then asked to rate their current menstrual pain. After an MRI or ultrasound scan, participants ingested the highest starting dose of naproxen sodium recommended by the manufacturer (440 mg; Bayer Co., Whippany, NJ, USA) with water. During a subsequent 90-minute wait time, participants filled out questionnaires using an iPad to obtain complete medical, surgical, psychological, gynecological, and obstetrical history. Included in these measures was a self-rating of menstrual pain over the past three months with and without painkillers on a 0–100 mm visual analog scale (VAS). We verified that participants reporting a history of endometriosis had prior surgical confirmation via their medical records. Additionally, we confirmed other potential contributing anatomical factors (e.g., adenomyosis, leiomyoma) with MRI. After the 90-minute wait time, participants re-assessed their menstrual cramping pain on the NRS, and MRI/US scans were repeated. Blood Collection and Serum Retrieval Blood samples were collected by a trained research nurse using BD Vacutainer serum tubes (BD and Co., Franklin Lakes, NJ, USA). Blood samples were collected ∼90–125 minutes following naproxen ingestion; this is within the time range where circulating naproxen concentrations are the highest [19]. Samples stayed at room temperature for 30–60 minutes to allow the blood to clot. Samples were then centrifuged at 3,000 × g for 5–10 minutes at 0°C to retrieve the serum supernatant. Serum samples were immediately aliquoted into cryogenic vials (Corning Inc., Corning, NY, USA) and stored at –20°C until needed. Naproxen and ODN Analyses Serum naproxen and ODN concentrations were measured with Agilent 1290 UPLC coupled to a SCIEX QTRAP 6500 mass spectrometry system (AB SCIEX, Vaughan, Canada). For solid-phase extraction, 100 μL of each sample was mixed with 50 μL of stock internal standards and 350 μL of methanol, vortexed thoroughly, and stored in –20°C to precipitate the molecules of interest. Samples were then centrifuged for 10 minutes at 1,200 × g and kept on ice until loaded onto a Luna Omega C18 Column (100 Å, 1.6 μm, 2.1 mm × 150 mm; Phenomenex, Inc., Torrance, CA, USA). Standards (Naproxen-D3 and Naproxen, Cayman Chemicals, Ann Arbor, MI, USA; S-O-Desmethylnaproxen and rac O-Desmethylnaproxen-D3, Toronto Research Chemicals, North York, ON, Canada) were dissolved in mass spectrometry grade acetonitrile/water (90:10, v/v with 0.1% formic acid). Naproxen standards generated a reliable curve (r = 0.999) with a limit of quantitation at 100 ng/mL. ODN standards also generated a reliable curve (r = 0.999) with a limit of quantitation at 50 ng/mL. Statistical Analyses Data were analyzed in Microsoft Excel and GraphPad Prism (GraphPad Software, La Jolla, CA, USA), and P < 0.05 was considered the threshold for significance. To prevent bias, researchers who analyzed the data did not conduct the study visits and were blinded to participant identity by having access to a de-identified data set. As this was a substudy, we used all available blood samples from previous studies [11, 12]. A post hoc sensitivity analysis for our primary hypothesis of whether there was a direct correlation between serum concentration of naproxen and analgesic effectiveness revealed that we could reliably (α = 0.05, β = 1– 0.8) detect an effect size of |r| ≥ 0.41. To evaluate whether menstrual pain impaired naproxen absorption or metabolism, we compared healthy controls with women with dysmenorrhea. A post hoc sensitivity analysis (α = 0.05, β = 1–0.8) suggested that we could detect whether naproxen absorption in healthy controls (N = 10) is greater than in women with dysmenorrhea (N = 23) with an effect size of d = 1.0, corresponding to a difference of 37 µg/mL. This is a sufficient level for detecting a meaningful difference, given that our average levels are three times this threshold and that a prior study examining naproxen efficacy in rheumatoid arthritis suggested that serum concentrations of ≥50 µg/mL are associated with clinical efficacy [7]. Additionally, post hoc sensitivity analysis confirmed that we could detect if women with primary dysmenorrhea (N = 13) have different levels of naproxen absorption compared with women with secondary dysmenorrhea (N = 10), with an effect size of d = 1.1 corresponding to a difference of 40 µg/mL. The therapeutic benefit from naproxen was calculated as the percent change from baseline: Analgesic effectiveness was based on Figure 4 from Cepeda’s study on acute pain relief descriptors [20]: A 50% reduction in pain was considered indicative of very much improvement, 49% to 35% was indicative of much improvement, and below 35% was indicative of a minimal improvement in pain. Unpaired Student t tests were used to compare group differences between healthy controls and participants with primary or secondary dysmenorrhea for demographic variables. Paired Student t tests were used to compare reported pain scores before and after taking naproxen. Linear regression analyses were used to evaluate the relationship between analgesic effectiveness and serum naproxen and serum ODN concentrations. All results are presented as mean ± standard error.

The demographic and clinical characteristics of healthy controls, participants with primary dysmenorrhea (Dys), and those with secondary dysmenorrhea (Dys-S) are listed in Table 1. Demographic characteristics were similar between the groups, except that Dys-S participants tended to be older (P < 0.001) and to have a higher body mass index (P = 0.023) when compared with Dys participants. According to the diary data on menses, Dys and Dys-S participants reported severe menstrual pain, unlike healthy controls (P < 0.001). Menstrual pain in Dys and Dys-S participants was severe enough to cause two or six missed school/work days in the last 90 days, respectively. Table 1. | HC (N = 12) | Dys (N = 13) | Dys-S (N = 10) | | |---|---|---|---| | Age, y | 28 ± 3 | 23 ± 2 | 32 ± 2# | | BMI, kg/m2 | 23 ± 1 | 21 ± 1 | 25 ± 2# | | Race/ethnicity | ||| | White | 8 (67) | 9 (70) | 9 (90) | | African American | 1 (8) | 2 (15) | 1 (10 ) | | Other | 3 (25) | 2 (15) | 0 (0) | | Oral contraceptive usage | 4 (33) | 2 (15) | 1 (10) | | Absenteeism, d/3 mo | 0 ± 0 | 2 ± 1* | 6 ± 2* | | Menstrual pain without NSAIDs (1–100) | 9 ± 2 | 73 ± 5*** | 76 ± 5*** | | Menstrual pain with NSAIDs (1–100) | 3 ± 1 | 23 ± 4*** | 60 ± 10***,## | | Gynecological diagnoses | ||| | Endometriosis | — | — | 8 (80) | | Leiomyoma | — | — | 7 (70)† | | Ovarian cysts | — | — | 2 (20) | Data are represented as mean ± standard error of the mean or number (%). BMI = body mass index; Dys = primary dysmenorrhea; Dys-S = secondary dysmenorrhea; HC = healthy control; NSAID = nonsteroidal anti-inflammatory drug. When compared with HC: P < 0.05; P < 0.01; P < 0.001. When compared with Dys: P < 0.05; P < 0.01. The seven participants with leiomyoma also had endometriosis. To characterize the analgesic effectiveness of naproxen in relation to absorption, we evaluated pain levels before and after naproxen administration. Before receiving naproxen at the study visit, both Dys (5.5 ± 0.6, NRS) and Dys-S (6.2 ± 0.6) participants reported greater maximum cramping pain than healthy controls (0 ± 0, P < 0.001). Ninety to 120 minutes following naproxen administration, Dys participants reported a significant pain reduction to 2.7 ± 0.6, or by 50% ± 11% (P = 0.002). In contrast, Dys-S participants’ pain only decreased to 4.9 ± 0.7, or by 23% ± 8% (P = 0.130). Concerned that pain could be a potential confounder of naproxen absorption and metabolism (as was observed with rheumatoid arthritis), we compared serum naproxen and ODN concentrations between dysmenorrheic participants and healthy controls. To approximate naproxen absorption and metabolism, we quantified serum naproxen and ODN concentrations. Serum naproxen concentrations should largely reflect gastrointestinal absorption, while serum ODN concentrations should reflect liver drug metabolism. There were no significant differences between the serum naproxen concentration of the combined dysmenorrheic cohort (126 ± 10 μg/mL) and healthy controls (135 ± 8 μg/mL, P = 0.459). Both groups had similar serum ODN concentrations as well (healthy controls: 355 ± 58 ng/mL; dysmenorrheic cohort: 381 ± 56 ng/mL; P = 0.753). Serum naproxen and ODN concentrations were correlated within the combined cohort (r = 0.77, P < 0.001) (Figure 1). Thus, low serum concentrations of naproxen are unlikely due to heightened metabolism. Given that dysmenorrhea was not a covariate of naproxen absorption or metabolism, we proceeded to examine naproxen effectiveness exclusively within the dysmenorrheic cohort. Analgesic effectiveness was calculated as a percent change in pain rating before and after naproxen administration. Whereas some participants had 100% analgesic effectiveness, other participants had 17% worsened pain after naproxen. A correlation analysis revealed that serum naproxen concentrations were associated with analgesic effectiveness (r = 0.49, P = 0.019) (Figure 2). Analgesic effectiveness was also associated with increased ODN concentrations (r = 0.45, P = 0.032). Accordingly, increased metabolism of naproxen to its metabolite ODN is unlikely responsible for impaired analgesic effectiveness. Reduced serum concentrations in less responsive participants were unlikely due to increased BMI because the correlation between BMI and serum concentration was weak (r = –0.20, P = 0.342). For ease of interpretation, we generated a table displaying the analgesic effectiveness of naproxen in relationship to serum concentrations (Table 2). Acute alleviation of menstrual pain (>50% improvement) was associated with an average naproxen concentration of 140 µg/mL; minimal responders (<35% improvement) had <100 µg/mL. Table 2. | % Improvement | No. | Dys, No. (%) | Dys-S, No. (%) | Naproxen, µg/mL | ODN, ng/mL | |---|---|---|---|---|---| | ≥50% pain relief | 8 | 7 (54) | 1 (10) | 145 ± 14 | 479 ± 114 | | 49–35% pain relief | 6 | 2 (15) | 4 (40) | 141 ±10 | 485 ± 107 | | <35% pain relief | 9 | 4 (31) | 5 (50) | 98 ± 19 | 224 ± 64 | Serum concentrations are represented as mean ± standard error of the mean. Dys = primary dysmenorrhea; Dys-S = secondary dysmenorrhea. During analysis, we noticed that one dysmenorrheic participant had extremely low serum naproxen and ODN concentrations. To confirm that her data did not skew the results, we conducted sensitivity analyses by removing her from the data set. Subsequent analyses showed that serum naproxen concentrations were still correlated to reported analgesic effectiveness (r = 0.44, P = 0.042). However, the association between serum ODN concentrations and reported pain relief was no longer significant (r = 0.41, P = 0.061). To assess the possible effects of uterine anatomical factors on drug absorption within the dysmenorrheic cohort, we also examined reported pain relief while considering dysmenorrhea status. While group differences were not significant, Dys-S participants had less analgesic benefit from naproxen (23% ± 8% pain reduction) when compared with Dys participants (50% ± 11%, P = 0.053). There was no significant difference in serum naproxen concentrations (Dys: 134 ± 14 µg/mL; Dys-S: 114 ± 14 µg/mL; P = 0.315). An additional multivariate regression analysis was performed adjusting for dysmenorrhea status as a potential confounder. In this multivariate model (r = 0.57, P = 0.021), lower serum naproxen concentrations remained associated with reduced pain relief (P = 0.038); dysmenorrhea status was not associated with reported pain relief (P = 0.128).

Main Findings This preliminary study demonstrates that some cases of treatment-refractory menstrual pain are associated with low serum naproxen concentrations. The comparison of naproxen and ODN concentrations suggests that treatment resistance is in part due to poor drug absorption and not increased drug metabolism. Interpretation Our findings show that lower serum naproxen concentrations were moderately correlated with minimal pain relief following naproxen administration. Thirty-nine percent of all dysmenorrheic participants experienced minimal or no pain relief after taking a standard over-the-counter dose of naproxen. This percentage is higher than observed in prior studies [13, 14]. When based on dysmenorrhea status, naproxen resistance was present among 50% of participants with secondary dysmenorrhea. Consistently, the few studies that have examined NSAIDs for the treatment of endometriosis found limited evidence of their analgesic benefit, despite their common clinical use [21]. Similarly, other studies have shown that higher serum or plasma naproxen concentrations are associated with better pain relief for arthritis [7, 8]. For example, Day et al. noted that <25% of participants with low serum naproxen concentrations (<69 µg/mL) reported improvement in arthritis pain [7]. In contrast, more than 75% participants with higher concentrations (73–100 µg/mL) reported improvement in arthritis pain. In the present study, serum concentrations ≥140 µg/mL were associated with therapeutic efficacy (Table 2). Therefore, higher concentrations may be needed to effectively reduce the acute uterine inflammation associated with dysmenorrhea than the steady-state concentrations used to treat arthritis. The magnitude of these results is intriguing, given the belief that patients with chronic inflammatory conditions such as arthritis require higher concentrations and dosages of NSAIDs than patients with acute pain conditions [22, 23]. Higher levels of COX-2 blockade of prostaglandin synthesis within the uterus may be necessary to alleviate menstrual pain. This hypothesis is consistent with earlier work showing a direct correlation between severity of menstrual pain and prostaglandin levels in menstrual effluent [24]. Similar limitations in pain relief with NSAIDs may generalize to other conditions. For example, it has been shown that the reduced serum concentration of ibuprofen was associated with less effective analgesia after third molar extraction [25]. A more recent meta-analysis suggested that formulations of ibuprofen with more rapid absorption are associated with better analgesia [26]. Thus, it is likely that our finding that impaired absorption contributes to treatment resistance with naproxen sodium generalizes to other NSAIDs such as ibuprofen. Our findings suggest that poor drug absorption contributes to NSAID-resistant dysmenorrhea. The mechanisms responsible for poor NSAID absorption, however, are not yet understood. We hypothesized that alterations in digestive function due to either severe pain [9, 27] or the menstrual cycle [28] may contribute to poor absorption or metabolism in women with NSAID-resistant dysmenorrhea. Our results showed that dysmenorrheic participants and healthy controls had similar serum naproxen and ODN concentrations, suggesting that menstrual pain does not affect drug absorption. Additionally, no dysmenorrheic participants had high serum ODN paired with low serum naproxen. The absence of hypermetabolism within the dysmenorrheic cohort also suggests that menstrual pain does not affect naproxen metabolism. Other hypothesized factors that may reduce naproxen absorption and result in lower serum concentrations are food consumption and gastrointestinal acidity. One study showed that food consumption slows the rate of naproxen absorption, leaving peak plasma concentrations unaffected [19]. A meta-analysis of multiple NSAIDs has suggested that taking analgesics with food reduces early concentrations, reducing efficacy [29]. Given that blood samples were collected within the window of peak serum concentration [19] and that all participants refrained from eating and drinking approximately two hours before naproxen administration, it is unlikely that food consumption affected the observed findings. A high gastrointestinal pH may increase naproxen solubility and absorption [19]. Thus, a highly acidic gastrointestinal environment may reduce NSAID absorption and subsequent analgesic efficacy. To bypass this, accompanying naproxen with a sodium bicarbonate antacid may optimize absorption [30]. Higher dosages may also translate into more absorbed naproxen, but caution must be exercised with dose escalation given the associated risks and unwanted side effects [31]. Aside from absorption and metabolism, other physiological mechanisms not examined in this study may contribute to naproxen resistance in dysmenorrheic women. In some women with treatment resistance, naproxen may fail to inhibit cyclooxygenase-mediated prostaglandin synthesis. This failure could be due to alterations in the enzyme’s binding site, blocking naproxen’s pharmacological target. Mechanistic research exploring aspirin resistance in platelet therapy uncovered several point mutations in cyclooxygenase-1 and -2 that may have overlapping functional implications in NSAID-resistant dysmenorrhea [32, 33]. Alternatively, a different inflammatory pain pathway independent of cyclooxygenase [2, 34] could mediate refractory menstrual pain in women with therapeutically relevant serum concentrations, making NSAID administration inconsequential. We did not identify impaired naproxen absorption or heightened metabolism in women with secondary dysmenorrhea compared with women with primary dysmenorrhea. Historically, lack of a response to NSAIDs for dysmenorrhea is thought to be a symptom of an underlying anatomical condition [2]. In the case of endometriosis, unrelieved pain after NSAIDs likely reflects peritoneal afferents being sensitized directly by the inflammatory ectopic endometriotic implants [35, 36]. Similarly, in leiomyoma there are neurovascular bundles within the pseudocapsule that could elicit neurogenic pain with hypercontractility via an NSAID independent pathway [37, 38]. As there are few systematic studies on mechanisms of NSAID resistance, future studies should include secondary dysmenorrhea. Strengths and Limitations The strengths of this pilot study include the examination of naproxen’s analgesic effectiveness for menstrual pain during a study visit and the use of analytical chemistry techniques to reliably measure serum naproxen and ODN. The limitations include our smaller sample size, no comparative studies with other NSAIDs, the lack of a diagnostic pelvic exam for all dysmenorrheic participants, and the lack of longitudinal pharmacokinetic data. Future studies should investigate gastrointestinal factors that may affect naproxen absorption and whether higher doses could reduce menstrual pain in otherwise nonresponsive participants.

In summary, this preliminary study demonstrated that dysmenorrheic women nonresponsive to naproxen had low serum concentrations. If the association between poor NSAID absorption and treatment resistance is confirmed in larger subsequent studies, clinicians may consider incorporating NSAID serum measurements into therapeutic protocols to guide drug or dosage modifications. The diminished therapeutic efficacy observed in ∼40% of dysmenorrheic women using a first-line medication indicates the need to re-evaluate and improve existing therapeutic strategies for menstrual pain. Future studies should explore whether malabsorption contributes to NSAID resistance for other pain conditions. Acknowledgments We would like to acknowledge G.F. Gebhart at NorthShore University HealthSystem/Pritzker School of Medicine for his valuable scientific and editorial advice. The Mass Spectrometry, Metabolomics, & Proteomics Facility from the Research Resources Center at the University of Illinois at Chicago performed the serum naproxen and ODN analyses. Funding sources: Eunice Kennedy Shriver National Institute of Child Health and Human Development HD081709 HD091502 and NorthShore University HealthSystem. Disclosure/conflict of interest: The authors report no conflicts of interest.

- 1. Grosser T, Woolf CJ, FitzGerald GA. Time for nonaddictive relief of pain. Science 2017;355(6329):1026–7. [DOI] [PubMed] [Google Scholar] - 2. Oladosu FA, Tu FF, Hellman KM. Nonsteroidal antiinflammatory drug resistance in dysmenorrhea: Epidemiology, causes, and treatment. Am J Obstet Gynecol 2018;218(4):390–400. [DOI] [PMC free article] [PubMed] [Google Scholar] - 3. Onundarson PT, Flygenring B. Oral anticoagulant monitoring: Are we on the right track? Int J Lab Hematol 2019;41(S1):40–8. [DOI] [PubMed] [Google Scholar] - 4. Bhandari S, Pereira DIA, Chappell HF, Drakesmith H. Intravenous irons: From basic science to clinical practice. Pharmaceuticals (Basel) 2018;11(3):82. [DOI] [PMC free article] [PubMed] [Google Scholar] - 5. Khoo SH, Winston A, Back DJ. Combining resistance and pharmacology for optimum patient care. Curr Opin HIV AIDS 2007;2(3):157–68. [DOI] [PubMed] [Google Scholar] - 6. Tennant F. Why oral opioids may not be effective in a subset of chronic pain patients. Postgrad Med 2016;128(1):18–22. [DOI] [PubMed] [Google Scholar] - 7. Day RO, Furst DE, Dromgoole SH, Kamm B, Roe R, Paulus HE. Relationship of serum naproxen concentration to efficacy in rheumatoid arthritis. Clin Pharmacol Ther 1982;31(6):733–40. [DOI] [PubMed] [Google Scholar] - 8. Dunagan FM, McGill PE, Kelman AW, Whiting B. Naproxen dose and concentration: Response relationship in rheumatoid arthritis. Br J Rheumatol 1988;27(1):48–53. [DOI] [PubMed] [Google Scholar] - 9. Jamali F, Kunz-Dober CM. Pain-mediated altered absorption and metabolism of ibuprofen: An explanation for decreased serum enantiomer concentration after dental surgery. Br J Clin Pharmacol 1999;47(4):391–6. [DOI] [PMC free article] [PubMed] [Google Scholar] - 10.Hewitt GD, Gerancher KD, American College of Obstetricians and Gynecologists’ Committee on Adolescent Health Care. ACOG Committee Opinion No. 760: Dysmenorrhea and endometriosis in the adolescent. Obstet Gynecol 2018;132(6):e249–58. [DOI] [PubMed] [Google Scholar] - 11. Hellman KM, Kuhn CS, Tu FF, et al. CINE MRI during spontaneous cramps in women with menstrual pain. Am J Obstet Gynecol 2018;218(5):506.e1–e8. [DOI] [PMC free article] [PubMed] [Google Scholar] - 12. Oladosu FA, Tu FF, Farhan S, Garrison EF, Steiner ND, Roth GE, et al. Abdominal skeletal muscle activity precedes spontaneous menstrual cramping pain in primary dysmenorrhea. Am J Obstet Gynecol 2018;219(1):91.e1–e7. [DOI] [PMC free article] [PubMed] [Google Scholar] - 13. Owen PR. Prostaglandin synthetase inhibitors in the treatment of primary dysmenorrhea. Outcome trials reviewed. Am J Obstet Gynecol 1984;148(1):96–103. [DOI] [PubMed] [Google Scholar] - 14. Marjoribanks J, Ayeleke RO, Farquhar C, Proctor M. Nonsteroidal anti-inflammatory drugs for dysmenorrhoea. Cochrane Database Syst Rev 2015;30;(7):CD001751. [DOI] [PMC free article] [PubMed] [Google Scholar] - 15. Vree TB, van den Biggelaar-Martea M, Verwey-van Wissen CP, Vree JB, Guelen PJ. Pharmacokinetics of naproxen, its metabolite O-desmethylnaproxen, and their acyl glucuronides in humans. Biopharm Drug Dispos 1993;14(6):491–502. [DOI] [PubMed] [Google Scholar] - 16. Hjermstad MJ, Fayers PM, Haugen DF, et al. Studies comparing numerical rating scales, verbal rating scales, and visual analogue scales for assessment of pain intensity in adults: A systematic literature review. J Pain Symptom Manage 2011;41(6):1073–93. [DOI] [PubMed] [Google Scholar] - 17. Harris PA, Taylor R, Thielke R, Payne J, Gonzalez N, Conde JG. Research electronic data capture (REDCap)—a metadata-driven methodology and workflow process for providing translational research informatics support. J Biomed Inform 2009;42(2):377–81. [DOI] [PMC free article] [PubMed] [Google Scholar] - 18. Downie WW, Leatham PA, Rhind VM, Wright V, Branco JA, Anderson JA. Studies with pain rating scales. Ann Rheum Dis 1978;37(4):378–81. [DOI] [PMC free article] [PubMed] [Google Scholar] - 19. Davies NM, Anderson KE. Clinical pharmacokinetics of naproxen. Clin Pharmacokinet 1997;32(4):268–93. [DOI] [PubMed] [Google Scholar] - 20. Cepeda MS, Africano JM, Polo R, Alcala R, Carr DB. What decline in pain intensity is meaningful to patients with acute pain? Pain 2003;105(1–2):151–7. [DOI] [PubMed] [Google Scholar] - 21. Brown J, Crawford TJ, Allen C, Hopewell S, Prentice A. Nonsteroidal anti-inflammatory drugs for pain in women with endometriosis. Cochrane Database Syst Rev 2017;1:CD004753. [DOI] [PMC free article] [PubMed] [Google Scholar] - 22. Meara AS, Simon LS. Advice from professional societies: Appropriate use of NSAIDs. Pain Med 2013;14(Suppl 1):S3–10. [DOI] [PubMed] [Google Scholar] - 23. Crofford LJ. Use of NSAIDs in treating patients with arthritis. Arthritis Res Ther 2013;15(S3):S2. [DOI] [PMC free article] [PubMed] [Google Scholar] - 24. Chan WY. Prostaglandins and nonsteroidal antiinflammatory drugs in dysmenorrhea. Annu Rev Pharmacol Toxicol 1983;23(1):131–49. [DOI] [PubMed] [Google Scholar] - 25. Laska EM, Sunshine A, Marrero I, Olson N, Siegel C, McCormick N. The correlation between blood levels of ibuprofen and clinical analgesic response. Clin Pharmacol Ther 1986;40(1):1–7. [DOI] [PubMed] [Google Scholar] - 26. Moore RA, Derry S, Straube S, Ireson-Paine J, Wiffen PJ. Faster, higher, stronger? Evidence for formulation and efficacy for ibuprofen in acute pain. Pain 2014;155(1):14–21. [DOI] [PubMed] [Google Scholar] - 27. Ouweland FA, Franssen MJ, Putte LB, Tan Y, Ginneken CA, Gribnau FW. Naproxen pharmacokinetics in patients with rheumatoid arthritis during active polyarticular inflammation. Br J Clin Pharmacol 1987;23(2):189–93. [DOI] [PMC free article] [PubMed] [Google Scholar] - 28. Wald A, Van Thiel DH, Hoechstetter L, et al. Gastrointestinal transit: The effect of the menstrual cycle. Gastroenterology 1981;80(6):1497–500. [PubMed] [Google Scholar] - 29. Moore RA, Derry S, Wiffen PJ, Straube S. Effects of food on pharmacokinetics of immediate release oral formulations of aspirin, dipyrone, paracetamol and NSAIDs - a systematic review. Br J Clin Pharmacol 2015;80(3):381–8. [DOI] [PMC free article] [PubMed] [Google Scholar] - 30. Neuvonen PJ, Kivistö KT. Enhancement of drug absorption by antacids. An unrecognised drug interaction. Clin Pharmacokinet 1994;27(2):120–8. [DOI] [PubMed] [Google Scholar] - 31. Arfè A, Scotti L, Varas-Lorenzo C, et al. Non-steroidal anti-inflammatory drugs and risk of heart failure in four European countries: Nested case-control study. BMJ 2016;354:i4857. [DOI] [PubMed] [Google Scholar] - 32. Bhatt DL, Grosser T, Dong J-F, et al. Enteric coating and aspirin nonresponsiveness in patients with type 2 diabetes mellitus. J Am Coll Cardiol 2017;69(6):603–12. [DOI] [PubMed] [Google Scholar] - 33. Weng Z, Li X, Li Y, Lin J, Peng F, Niu W. The association of four common polymorphisms from four candidate genes (COX-1, COX-2, ITGA2B, ITGA2) with aspirin insensitivity: A meta-analysis. PLoS One 2013;8(11):e78093. [DOI] [PMC free article] [PubMed] [Google Scholar] - 34. Hellman KM, Yu PY, Oladosu FA, et al. The effects of platelet-activating factor on uterine contractility, perfusion, hypoxia, and pain in mice. Reprod Sci 2017;25(3):384–94. 1933719117715122. [DOI] [PubMed] [Google Scholar] - 35. McAllister SL, Dmitrieva N, Berkley KJ. Sprouted innervation into uterine transplants contributes to the development of hyperalgesia in a rat model of endometriosis. PLoS One 2012;7(2):e31758. [DOI] [PMC free article] [PubMed] [Google Scholar] - 36. Williams C, Hoang L, Yosef A, et al. Nerve bundles and deep dyspareunia in endometriosis. Reprod Sci 2016;23(7):892–901. [DOI] [PubMed] [Google Scholar] - 37. Malvasi A, Cavallotti C, Morroni M, et al. Uterine fibroid pseudocapsule studied by transmission electron microscopy. Eur J Obstet Gynecol Reprod Biol 2012;162(2):187–91. [DOI] [PubMed] [Google Scholar] - 38. Sun Y, Zhu L, Huang X, Zhou C, Zhang X. Immunohistochemical localization of nerve fibers in the pseudocapsule of fibroids. Eur J Histochem 2014;58(2):2249:158–62. [DOI] [PMC free article] [PubMed] [Google Scholar]