Medroxyprogesterone acetate (MPA), a synthetic form of the naturally occurring steroid hormone progesterone widely used in contraceptives and hormone replacement therapy (HRT), increases aggression and anxiety and reduces sexual activity in female monkeys, according to a study published in the June edition of The Journal of Clinical Endocrinology and Metabolism. The investigators, from the Yerkes National Primate Research Center of Emory University and the Center for Behavioral Neuroscience (CBN) in Atlanta, say the findings may explain anecdotal reports of mood changes, depression and loss of libido in some women who use MPA for contraception and HRT.