FDA Needs to Ensure More Study of Gender Differences in Prescription Drug Testing

Drug therapy is the most common and one of the most important forms of medical treatment used for men and women. Because of physiological differences, however, men and women can respond differently to the same prescription drug. %or example, women tend to metabolize some antihypertensive and cardiovascular drugs at a slower rate than men. Also, drug interactions with women’s hormones and women’s use of oral contraceptives during their childbearing years can cause different responses. Despite evidence of important differences in the way gender can affect drug response, drug manufacturers may not be studying drug test data for possible gender-related differences.

Given the potential for different responses to drugs based on gender, you expressed concern that women could be at risk if the Food and Drug Administration (FDA) approves drugs on the basis of clinical trials’ in which women were underrepresented. At your request, we examined FDA'S policies and the pharmaceutical industry’s practices regarding research on women in prescription drug testing.

We reviewed FDA'S policy guidance for drug manufacturers and interviewed FDA, National Institutes of Health (NIH), and Institute of Medicine officials; pharmaceutical representatives; and experts in pharmacology. We also performed an extensive literature search on topics related to drug testing and clinical trials.

In our examination of drug manufacturers’ testing practices in the United States, you asked us to provide information on the prescription drugs FDA approved over a recent 3-l/2-year period. Specifically, we were to determine (1) the representation of women in drug testing, (2) the sufficiency of female participation in drug trials to assesssignificant gender-related differences, (3) the extent to which trial data were analyzed for differences in response related to gender, and (4) whether studies were conducted to examine drug interaction with the varying hormonal status of women and oral contraceptive use. 

FDA guidance to drug manufacturers recommends that they test new drugs on representative patient populations. FDA, however, does not define “representative,” and manufacturers are not consistent in their application of FDA'S guidance. A quarter of the drug manufacturers in an industry survey reported that they do not deliberately recruit representative numbers of women as participants in drug trials. Further, more than half said that FDA asked them to include women in drug trials, but the remainder said they had not been asked.

Women were included in clinical trials for all the drugs in our survey but were generally underrepresented in those trials. Our standard of representativeness is a comparison of the proportion of women among clinical trial participants with the proportion of women among those persons with the disease for which the drug is intended. Using this approach, we determined that for more than 60 percent of the drugs, the representation of women in the test population was less than the representation of women in the population with the corresponding disease.

Although women may not be proportionately represented in trials for some drugs, there were enough to detect gender-related differences in response for most drugs in our survey. The absolute number of women in clinical drug trials is a key determinate of whether manufacturers can detect significant differences in response that may be related to gender, according to FDA. We observed, however, that while the trials supporting most drugs did include at least 250 women, the minimum number suggested by FDA, for about a third of the drugs, fewer than 250 women were included as trial participants.

Even when enough women are included in drug testing, often trial data are not analyzed to determine if women’s responses to a drug differed from those of men. Also, many drug manufacturers do not study whether their drugs specifically interact with the hormones present in women, including hormones commonly found in oral contraceptives. This lack of knowledge about gender-related differences in drug response can create a critical gap in information about how best to tailor drug therapies to women.