From Mad Gender Science!

An antagonist is a drug which binds to a receptor without activating it. This results in the drug competing with and displacing agonists from the receptor and thereby preventing the receptor from being activated. Antagonists of the androgen receptor like spironolactone, cyproterone acetate, and bicalutamide are used as antiandrogens and prevent the activation of the receptor by androgens like testosterone and dihydrotestosterone.