Progesterone capsules may interfere with the effects of bromocriptine and may raise the plasma concentration of ciclosporin. Progesterone capsules may affect the results of laboratory tests of hepatic and/or endocrine functions. Metabolism of progesterone capsules is accelerated by rifamycin an antibacterial agent. The metabolism of progesterone by human liver microsomes was inhibited by ketoconazole (IC <0.1 μM). Ketoconazole is a known inhibitor of cytochrome P450 3A4. These data therefore suggest that ketoconazole may increase the bioavailability of progesterone. The clinical relevance of the in vitro findings is unknown.