Cimetidine blocks testosterone synthesis.

Although an assortment of endocrine dysfunctions are reported in men treated with cimetidine hydrochloride, gonadotropin and sex steroid levels are usually normal. A 66-year-old man with previously normal sexual function presented with breast tenderness and sexual dysfunction after receiving cimetidine. His plasma testosterone level was low, and elevated gonadotropin levels suggested a primary testicular disorder. His sexual dysfunction improved and his plasma testosterone level rose to normal after cimetidine therapy was discontinued. Readministration of cimetidine therapy resulted in the prompt recurrence of his sexual problems and low testosterone level. A reversible defect in 17-beta-hydroxysteroid dehydrogenase is the mechanism postulated to have caused his low testosterone level. While the frequency of this abnormality is uncertain, its reversible nature and the wide use of cimetidine make it an important cause of sexual dysfunction in men.