Analgesic Use and Renal Function in Men FREE

Nonnarcotic analgesics are among the most widely used medications and are used at least weekly by one quarter of US adults.^1- 3 High acute doses of analgesics, particularly nonsteroidal anti-inflammatory drugs (NSAIDs), have been implicated as causes of acute renal failure,^4- 7 which is most commonly seen among patients with impaired renal function. It has also been suggested that prolonged use of various analgesics may cause progressive renal damage ultimately leading to chronic kidney failure. The earliest study linking chronic analgesic use and chronic renal dysfunction was reported almost 50 years ago.⁸ In the ensuing half century, numerous case reports, several case-control studies, and 2 small cohort studies have raised the possibility of an association between use of various analgesics and chronic renal disease.

With respect to different types of analgesics, the association between phenacetin and kidney disease has been quite strong.^9- 11 In the United States, phenacetin was banned from the market in 1983.¹² Whether other analgesics are implicated in renal dysfunction is less clear, with some studies showing no association between regular use of analgesics such as acetaminophen, aspirin, and other NSAIDs and chronic renal dysfunction^9,13 and other studies showing increased risk.^{10- 11,14- 16} A recent case-control study, for example, reported a 2-fold increased risk of end-stage renal disease among individuals with lifetime use of more than 1000 acetaminophen pills and an 8-fold increased risk among those with a lifetime cumulative dose of more than 5000 NSAID pills.¹⁶

These case-control studies, however, have significant methodologic limitations, including difficulty in establishing the time order of exposure, confounding by indication, selection bias, and recall bias.^17- 18 Given these methodologic limitations, the existing evidence is insufficient to support a causal association.

One way of minimizing confounding by indication and recall bias inherent in the case-control design is to examine the effect of long-term analgesic use on earlier, preclinical renal disease outcomes in a large cohort study. The Physicians' Health Study (PHS) cohort provided such an opportunity. In this group of more than 11 000 initially healthy men who provided blood samples, we evaluated the association between retrospective self-reported intake of acetaminophen, aspirin, and other NSAIDs and renal function, as measured by creatinine levels and creatinine clearance.

The design and methods of the PHS are described in detail elsewhere.¹⁹ In brief, the study consisted of 22 071 apparently healthy male physicians without prior history of cardiovascular disease, cancer (except nonmelanoma skin cancer), current liver disease or renal dysfunction (defined as renal failure or insufficiency), or other major illnesses. They were randomly assigned to 1 of 4 groups: aspirin (325 mg on alternate days), beta carotene (50 mg on alternate days), both, or neither to test whether aspirin and/or beta carotene prevent heart disease or cancer. In January 1988, the study's aspirin arm was terminated early based primarily upon a statistically extreme 44% reduction in the occurrence of a first myocardial infarction among those randomized to aspirin. The trial's beta carotene arm was continued until its scheduled completion in December 1995.

Every 6 months for the first year and annually thereafter participants were sent follow-up questionnaires. Information was collected regarding study compliance; personal characteristics, including weight and height; medical history, including history of hypertension, elevated cholesterol, and diabetes; health behaviors, including smoking status (never, past, current), frequency of physical activity (none, monthly, 1-4 times per week, ≥5 times per week), and use of alcohol (rarely/never, monthly, weekly, daily, or more); and the occurrence of cardiovascular disease and any other relevant medical events during the study period.

At the end of the randomized trial, the physicians completed a detailed retrospective history of analgesic use over the 14-year study period. Participants were asked about their use of 8 categories of analgesics: (1) aspirin and buffered aspirin alone (excluding study-drug aspirin), (2) combination pills containing primarily aspirin, (3) acetaminophen alone, (4) combinations containing primarily acetaminophen, (5) combinations containing both acetaminophen and aspirin, (6) other NSAIDs, (7) other pain relief agents used as single agents, and (8) other pain relief combinations. For each category, participants were asked if they had taken this type of medication 12 or more times since enrolling in the PHS in 1982. Those answering yes were asked to indicate how many days per month and how many pills per day these medicines were taken on average in the preceding year. The participants were also asked if this pattern had been constant since enrolling in the PHS. Those answering no were asked to record the number of years for which this had been the pattern of use and to describe their earlier pattern. Aspirin use as part of the PHS was calculated by using the compliance data reported on each participant's annual questionnaire and his assignment to aspirin or placebo. After the aspirin arm of the PHS was terminated, participants could elect to continue receiving study aspirin; their reported compliance with this aspirin was also included. Total aspirin use was the sum of aspirin provided as part of the PHS and self-reported aspirin use on the analgesic-history questionnaire.

Based on this information, analgesic consumption was categorized as acetaminophen (acetaminophen and acetaminophen-containing compounds), aspirin (aspirin and aspirin-containing compounds), and other NSAIDs. We did not include in these analyses the use of combination pills containing aspirin and acetaminophen nor the use of other analgesics, because the number of users of these was small (237 for aspirin and acetaminophen, 202 for other analgesic and other analgesic combinations). For each category of analgesic use, the number of pills per year was recorded and the cumulative number of pills over the study period was calculated by summing the reported use over the years of the study. In order to simplify comparisons with prior studies,¹⁶ we created 4 categories of analgesic use: never (<12 pills during the 14-year study period), 12 to 1499 pills, 1500 to 2499 pills, and 2500 or more pills. Very high use (>7000 pills over the study period) was also examined in secondary analyses.

To assess internal validity, information on analgesic use was obtained using a structured telephone interview from a sample of 910 randomized PHS participants without a history of renal or other major diseases 1 to 2 years after they completed the written analgesic-use questionnaire. These participants were asked about their use of over-the-counter and prescription analgesics, the average number of days per month and pills per day these medications were used in the previous year and in the 2 prior 5-year periods, and use of higher doses of analgesics for at least 1 month or more. Cumulative analgesic use assessed by telephone interviews was compared with that reported on the written questionnaire. The correlation coefficients of analgesic use during an identical follow-up period from 1996 until the return of the written questionnaire were 0.76 for acetaminophen, 0.70 for aspirin, and 0.45 for other NSAIDs. For the period from 1988 to 1995 the correlations between the 2 methods were 0.67 for acetaminophen, 0.40 for aspirin, and 0.46 for other NSAIDs. The correlations for analgesic use are of similar magnitude to correlations over several years for variables widely used in epidemiologic studies such as blood pressure²⁰ and serum cholesterol.²¹ In comparing the respondents in the highest quartiles on both questionnaires, there was a concordance of 51.8% for acetaminophen, 63.8% for aspirin, and 48.4% for other NSAIDs. Among those in the highest quartile of analgesic use on the written questionnaire, the concordance for being in the 2 highest quartiles on the telephone questionnaire was 75.3% for acetaminophen, 86.2% for aspirin, and 64.8% for other NSAIDs.

Blood kits with vacuum tubes containing EDTA, complete instructions for blood draws, and coldpacks were mailed to all enrolled physicians between December 1995 and October 1997. Each was asked to have a blood sample drawn into the vacuum tube and to return the sample in the coldpack by prepaid overnight courier. Upon receipt, each sample was centrifuged, divided into aliquots, and stored at − 82°C. None of the samples inadvertently thawed during storage. Follow-up samples were received from 11 531 participants; creatinine analyses were available for 11 106 of these participants.

Blood samples were analyzed at Oxford University, England, using an automated Jaffe rate method on a SYNCHRON LX20 autoanalyzer (Beckman Coulter, Fullerton, Calif) for quantification of creatinine. Plasma creatinine is stable in chilled next-day whole blood samples preserved with EDTA.²² To assess quality control, splits were submitted; the coefficient of variation for these samples was 7.07%. The difference in means between the study samples and the repeat quality control samples was 0.018 mg/dL (SD, 0.67). Intrabatch coefficients of variation on internal quality control runs were 1.4% to 3.6%.

The primary outcomes of interest were an elevated creatinine level, defined as 1.5 mg/dL (133 µmol/L) or higher at the time of the follow-up blood collection, and a reduced creatinine clearance, defined as 55 mL/min (0.9 mL/s) or less. Creatinine clearance was calculated using the Cockcroft-Gault equation: CrCl = [(140 − age) × (weight in kilograms)] / [72 × (creatinine in mg/dL)].²³

Creatinine data were available for 11 106 participants. To avoid recall bias and confounding by indication, we excluded 71 participants who reported renal insufficiency or renal failure prior to the completion of the analgesic assessment questionnaire, and 3 participants with creatinine levels greater than 5.0 mg/dL (442 µmol/L) at the time of creatinine measurement, leaving 11 032 participants for analysis.

Characteristics of participants with elevated creatinine level and reduced creatinine clearance were compared with t test or χ² test. Comparisons of mean creatinine level and creatinine clearance were computed by the general linear models procedure (SAS version 6.12, SAS Institute, Cary, NC), adjusted for age. These linear models also provided estimates of statistical significance for categories of analgesic intake controlling for age. Multiple logistic regression analysis was performed to adjust simultaneously for significant risk factors for elevated creatinine levels or reduced creatinine clearance, including age; body mass index (BMI); history of hypertension, elevated cholesterol, or diabetes at baseline or during the study period; occurrence of cardiovascular disease during the study period; physical activity; and other analgesic use. For each relative risk (RR), 2-tailed P values and 95% confidence intervals (CIs) were calculated. Values of P for trend were calculated across categories. Logistic regression models were validated using the method of Hosmer and Lemeshow,²⁴ and there was no evidence of a lack of fit according to the goodness-of-fit statistic. Because of concerns about residual confounding by BMI, analyses using creatinine clearance were additionally stratified by BMI categories (BMI <30 and ≥30 kg/m²). Subgroup analyses comparing men aged 60 years and older vs those younger than 60 years, as well as those with a history of hypertension and/or diabetes at baseline, were also performed.

The mean creatinine levels and creatinine clearance of the 11 032 men with no prior history of renal failure or insufficiency for whom follow-up creatinine measures were available are presented by age categories in Table 1. As expected, age was strongly associated with both outcomes. In blood samples collected an average of 14 years after study enrollment, mean creatinine level was 19% higher and mean creatinine clearance was 50% lower among men aged 70 to 84 years at baseline than among men aged 40 to 49 years. Due to the profound, nonlinear effect of age on renal function, all subsequent data have been adjusted for age by 5-year age categories.

The age-adjusted baseline characteristics of the study cohort according to creatinine levels and creatinine clearance categories are presented in Table 2. Elevated creatinine levels, defined as 1.5 mg/dL (133 µmol/L) or greater, was noted in 460 participants (4.2% of the study cohort). Participants with elevated creatinine levels were significantly older, had higher BMIs, and were more likely to have had a history of hypertension, elevated cholesterol, or the occurrence of cardiovascular disease during the study. Mean age-adjusted use of acetaminophen, aspirin, and other NSAIDs did not differ significantly between those with and without elevated creatinine level. Reduced creatinine clearance was present in 1258 participants (11.4%). Men with reduced creatinine clearance were significantly older and leaner and were more likely to have had a history of hypertension at baseline and the occurrence of cardiovascular disease during the study. Mean age-adjusted use of acetaminophen was lower in men with reduced creatinine clearance while age-adjusted means did not differ for aspirin or other NSAID use.

We examined mean creatinine levels and creatinine clearance according to 4 predetermined categories of analgesic exposure over the 14-year follow-up (Table 3). Compared with nonusers, men who used 12 to 2499 acetaminophen pills had lower mean creatinine levels, although the absolute differences were small (0.01 mg/dL to 0.02 mg/dL). For users of aspirin and other NSAIDs, mean creatinine level did not differ significantly across analgesic use categories. Mean creatinine clearance increased significantly across increasing use categories of both acetaminophen and other NSAIDs, but did not differ significantly across aspirin use categories. Because weight is part of the formula for calculating creatinine clearance and analgesic use tends to increase with increasing weight, we conducted analyses stratified by BMI to evaluate possible confounding by weight. Stratifying by BMI minimized confounding by weight and attenuated the protective inverse trends for acetaminophen and other NSAIDs.

We performed multivariable analyses that adjusted for potential renal risk factors (age; BMI; history of hypertension, elevated cholesterol, and diabetes; occurrence of cardiovascular disease during the study; and physical activity) as well as for use of other analgesics (Table 4). Acetaminophen use was associated with a slightly decreased risk of elevated creatinine levels (P for trend = .05), and intake of 2500 or more acetaminophen pills over the study period was associated with an RR of elevated creatinine levels of 0.83 (95% CI, 0.50-1.39). For aspirin, no significant trend for creatinine levels was observed (P for trend = .96), and intake of 2500 or more pills was associated with an RR of 0.98 (95% CI, 0.53-1.81). For other NSAIDs, no significant trend was observed (P for trend = .86), and intake of more than 2500 pills was associated with an RR of 1.07 (95% CI, 0.71-1.64). When we examined men who consumed 7000 or more pills of each analgesic during the study (>500 pills/year) the RRs of elevated creatinine levels were 0.51 (95% CI, 0.16-1.67) for acetaminophen, 1.35 (95% CI, 0.57-3.18) for aspirin, and 1.61 (95% CI, 0.90-2.90) for other NSAIDs. Results did not differ when the 74 men with reported renal disease were included.

To identify a possible association with analgesic use among older men, who might be most susceptible to renal dysfunction, we examined the association between analgesic use and risk of elevated creatinine levels among men aged 60 years or older at baseline. Even in this high-risk group, no significant trends of increased risk of elevated creatinine levels were observed for any of the analgesic use categories. When the 2422 men with a diagnosis of hypertension or diabetes at baseline were examined separately, the RRs of elevated creatinine levels with consumption of 2500 or more pills were 0.70 (95% CI, 0.26-1.83) for acetaminophen, 0.57 (95% CI, 0.35-0.94) for aspirin, and 0.63 (95% CI, 0.27-1.44) for other NSAIDs.

Multivariable models were also fitted to examine the association between analgesic exposure and reduced creatinine clearance while adjusting for potential confounders. No significantly increased risks were observed for any of the 3 analgesic categories (Table 5). Results were similar for men aged 60 years or older, as well as for those with a history of hypertension or diabetes at baseline. Additional multivariable analyses were also performed (see online Table A 

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In this large cohort study of initially healthy male physicians followed up for an average of 14 years, we found no significant associations between the use of acetaminophen, aspirin, or other NSAIDs and either elevated creatinine levels or reduced creatinine clearance even among men who used 2500 or more pills (an average of 3 to 4 pills/week) over the study period. Established predictors of renal dysfunction, including age and hypertension, were generally associated with elevated creatinine levels and reduced creatinine clearance. Diabetes rates tended to be higher among those with elevated creatinine levels and reduced creatinine clearance.

Our study is the largest cohort analysis to date of the association between analgesic use and risk of renal dysfunction. Two small cohort studies^25- 26 both found increased risks of renal dysfunction, mortality, and cancer associated with use of phenacetin. Data from both of these investigations, however, indicate that the analgesic users had more underlying illnesses than the nonusers, and these illnesses may have led both to increased analgesic use and to the wide range of outcomes identified in respective studies.¹⁷

Our data contrast with findings from most of the earlier case-control studies. Since 1980, at least 8 case-control studies have examined the association between analgesic use and various measures of chronic renal failure, including end-stage renal disease,^11,13- 16 renal papillary necrosis,⁹ and first diagnosis of chronic renal disease.^10,27 Two of these^9,13 showed no clear associations, while the others found positive associations.

Limitations in the case-control design make it particularly difficult to establish a causal relationship between analgesic use and renal dysfunction. Studies that included primarily patients with established renal failure cannot determine whether analgesic use predated renal failure or was a consequence of renal dysfunction. Once renal disease is discovered, patients are generally counseled to use acetaminophen for their analgesic needs instead of aspirin or other NSAIDs. In our study, the use of a preclinical end point, the collection of exposure data prior to assessment of renal function, and the exclusion of participants with a diagnosed history of renal disease should substantially minimize the likelihood of confounding by indication, selection bias, and recall bias.

Measuring long-term use of over-the-counter medications is a challenging task. The structured questionnaire we used to assess analgesic consumption was similar to the methods used by Perneger et al¹⁶ and other investigators in case-control studies that found significant increased risks of renal failure with analgesic exposures. Furthermore, we found moderate reliability and internal validity when comparing estimates of analgesic use obtained by written questionnaires with those obtained via structured telephone interviews, making it unlikely that measurement error explains the null association we observed.

The large size of the PHS cohort made it possible to examine and detect even modest associations between analgesic use and the risk of elevated creatinine levels or reduced creatinine clearance. Furthermore, while most of the PHS participants reported moderate use of analgesics, this cohort study included more individuals reporting high use of analgesics (≥2500 pills) than any prior case-control study. Over the 14-year study period, 474 men reported high use of acetaminophen, 2360 reported high use of aspirin, and 816 reported high use of other NSAIDs. In contrast, the numbers of high users (cumulative lifetime use of ≥1000 pills) in the largest case-control study to date were 139 for acetaminophen, 173 for aspirin, and 52 for other NSAIDs.¹⁶ Given the large number of high users in our cohort, and the relatively large risks suggested by case-control studies,¹⁶ we should have been able to detect an increased risk of renal dysfunction.

Our study has several limitations. The PHS cohort is composed of relatively healthy men, most of whom are white. Thus, we cannot extend these results to women or minority populations. Retrospective assessment of analgesic use within an ongoing cohort study made it impossible to obtain exposure information from participants who died before the analgesic assessment. However, given that only 14 deaths with renal failure as a primary or contributing cause of death were confirmed over the 14-year study, this small loss of follow-up is unlikely to alter the results of our analyses. We used a retrospective assessment of analgesic exposure that was not designed to assess the short-term effects of high analgesic use but to examine the association with cumulative use that had been raised by prior studies. Error in measuring both analgesic use and creatinine would tend to minimize a relationship. In addition, our questionnaire assessed pill use rather than dose. However, data from our validation study indicate that most men used standard over-the-counter doses (50.7% of participants took 325-mg acetaminophen tablets, 78.6% took 325-mg aspirin tablets, and 70.3% took 200-mg ibuprofen tablets).

The use of a preclinical end point is another possible limitation. While this has the advantage of avoiding recall bias and changes in medication based on knowledge of renal disease, it assumes that the relationship between analgesic use and renal disease is cumulative and linear, rather than requiring a high threshold effect. Furthermore, we specifically excluded individuals with a history of renal dysfunction to avoid problems of recall and indication bias, so we cannot directly examine the impact of analgesic exposure among individuals with preexisting renal impairment. However, we found no increased risk among higher-risk groups such as those aged 60 years or older or those with hypertension or diabetes at baseline, and our results did not differ when the 74 men with prior history of renal disease were included. Because the PHS tested the effects of aspirin, this cohort study included fewer never users of aspirin. Nevertheless, we should have been able to detect an adverse effect of this agent.

Because analgesic use is so common, even small increases in the RR of renal dysfunction could have a significant impact on rates of renal disease in the United States. This study, however, provides reassuring evidence that there does not appear to be a strong association between chronic analgesic use and chronic renal dysfunction among a large cohort of men. The findings from prior studies suggesting increased risk of renal failure with high cumulative acetaminophen use may have been due to limitations inherent in the case-control design.

While more research is needed, we believe that these findings provide reassurance to clinicians and patients that moderate analgesic use is unlikely to contribute to increased risk of renal dysfunction among individuals without a history of renal impairment.