Cyproterone is an anti-androgen. It blocks androgen receptors, preventing testosterone and other androgens from having their effects. By blocking those receptors, it reduces the amount of testosterone in the body through a mechanism called negative feedback. Cyproterone is chemically similar to progesterone and has some progesterone-like effects as well. Outside of transgender care it’s also used for prostate cancer, as combination antiandrogen and hormonal birth control for cis women (e.g., Dianette), and for chemical castration of sex offenders.