Cimetidine use and risk of prostate and breast cancer.

Histamine (H2) receptor antagonists, such as cimetidine and ranitidine, became available in the late 1970s and presently number among the most commonly used drugs. Cimetidine has been hypothesized to exert a cancer preventive effect on the prostate due to its ability to inhibit the binding of dihydrotestosterone to androgen receptors. Other hormonal effects of this drug include increases in serum prolactin levels and inhibition of 2-hydroxylation of estradiol. We assessed risk of prostate and breast cancers in a cohort of 48,512 members of the Group Health Cooperative of Puget Sound prescribed cimetidine or another H2 blocker between 1977 and 1995. Standardized incidence ratios were calculated comparing the observed numbers of cancers to those expected based on population rates in western Washington State. Because cimetidine, but not other H2 blockers, influences hormonal activity and metabolism, we conducted nested case-control studies comparing cancer risk among individuals treated with cimetidine to individuals who used other H2 blockers. Risks of breast and prostate cancers were identical among users of cimetidine and users of other H2 blockers (relative risk, 1.0 for both cancers). We observed no trend in risk of breast cancer according to time since first or last cimetidine prescription or number of cimetidine prescriptions filled. For prostate cancer, our findings were similar save for a modest increase in risk among men who had filled > or =21 cimetidine prescriptions (relative risk, 1.4; 95% confidence interval, 1.0-1.9). Our results suggest that use of cimetidine does not influence risk of female breast cancer. Further, these data provide little evidence to support the previously hypothesized preventive effect of cimetidine on risk of prostate cancer.