Cyproterone acetate (Androcur, Cyprostat, Cyproteron, Procur, Cyprone, Cyprohexal, Ciproterona, Cyproteronum, Neoproxil, Siterone) is an antiandrogen, i.e. it suppresses the actions of testosterone (and its metabolite dihydrotestosterone) on tissues. It acts by blocking androgen receptors which prevents androgens from binding to them and suppresses luteinizing hormone (which in turn reduces testosterone levels). Its main indications are prostate cancer, benign prostatic hyperplasia, priapism, hypersexuality and other conditions in which androgen action maintains the disease process. Due to its anti-androgen effect, it can also be used to treat hirsutism, and is a common component in hormone therapy for male-to-female transgendered people.

Until the development of leuprolide, cyproterone was one of the few drugs used to treat precocious puberty. It was also used in animal experimentation to investigate the actions of androgens in fetal sexual differentiation.

In addition, cyproterone acetate has weak progestational activity (e.g., it acts like progesterone) and can be used to treat hot flushes. As part of some contraceptive pills (Diane) it decreases acne and hirsutism (male-pattern hair growth).