An antiandrogen, or androgen antagonist, is any of a group of hormone antagonist compounds that are capable of preventing or inhibiting the biologic effects of androgens, male sex hormones, on normally responsive tissues in the body (see androgen insensitivity syndrome). Antiandrogens usually work by blocking the appropriate receptors, competing for binding sites on the cell's surface, obstructing the androgens' pathway.

Antiandrogens are often indicated to treat severe male sexual disorders, such as hypersexuality (excessive sexual desire) and sexual deviation, specifically paraphilias, as well as use as a antineoplastic agent and palliative, adjuvant or neoadjuvant hormonal therapy in prostate cancer.

Antiandrogens can also be used for treatment of benign prostatic hyperplasia (prostate enlargement), acne, androgenetic alopecia (male pattern baldness), and hirsutism (excessive hairiness). They are also occasionally used as a male contraceptive agent, to purposefully prevent or counteract masculinisation in the case of transwomen undergoing gender reassignment therapy, and to prevent the symptoms associated with reduced testosterone, such as hot flushes, following castration.

The administration of antiandrogens in males can result in slowed or halted development or reversal of male secondary sex characteristics, reduced activity or function of the accessory male sex organs, and hyposexuality (diminished sexual desire or libido).

The term antiandrogen withdrawal response (AAWR) describes the medical course taken when cancer cells adapt to feed on the antiandrogens rather than androgen, so that treatment must be halted in order to starve those cells thriving on the antiandrogens.

Currently available antiandrogen drugs (brand names in parentheses) include:

• Spironolactone (Aldactone, Spiritone), a synthetic 17-spirolactone corticosteroid, which is a renal competitive aldosterone antagonist in a class of pharmaceuticals called potassium-sparing diuretics, used primarily to treat low-renin hypertension, hypokalemia, and Conn's syndrome.
• Cyproterone acetate (Androcur, Climen, Diane 35, Ginette 35), a synthetic steroid, a potent antiandrogen that also possesses progestational properties.
• Flutamide (Eulexin), nilutamide (Anandron, Nilandron) and bicalutamide (Casodex), nonsteroidal, pure antiandrogens. Flutamide is the oldest and has more unwanted side effects than the others. Bicalutamide is the newest and has the least side effects.
• Ketoconazole (Nizoral), an imidazole derivative used as a broad-spectrum antifungal agent effective against a variety of fungal infections, side effects include serious liver damage and reduced levels of androgen from both the testicles and adrenal glands. Ketoconazole is a relatively weak antiandrogen.
• Finasteride (Proscar, Propecia) and dutasteride (Avodart), inhibitors of the 5-α-reductase enzyme that prevent the conversion of testosterone into dihydrotestosterone (DHT). Finasteride blocks only 5-α-reductase type II, dutasteride also blocks type I. They are not general antiandrogens in that they don't counteract the effects or production of other androgens than DHT.