Cimetidine is a known inhibitor of many isozymes of the cytochrome P450 enzyme system[14] (specifically CYP1A2, CYP2C9, CYP2C19, CYP2D6, CYP2E1, and CYP3A4). This inhibition forms the basis of the numerous drug interactions that occur between cimetidine and other drugs. For example, cimetidine may decrease metabolism of some drugs, such as those used in hormonal contraception. Cimetidine interferes with metabolism of the hormone estrogen, enhancing estrogen activity. In women, this can lead to galactorrhea, whereas in men, gynecomastia has been reported;[15] during postmarketing surveillance in the 1980s, cases of male sexual dysfunction were also reported.[16][17] Cimetidine also affects the metabolism of methadone, sometimes resulting in higher blood levels and a higher incidence of side effects, and may interact with the antimalarial medication hydroxychloroquine.[18]