Black cohosh (Actaea racemosa L. [syn. Cimifuga racemosa L.]) extracts (BCE) are marketed worldwide for the management of menopausal symptoms. However, recently more than 75 cases of hepatotoxicity associated with black cohosh ingestion have been reported. While these cases have not been fully substantiated for causality, the data suggest that herb-drug interactions may be involved rather than a direct hepatotoxic event. This work describes the in vitro inhibition of four CYP450 enzymes (1A2, 2D6, 2C9, 3A4) by black cohosh extracts and identifies the active inhibitory constituents. Ethanol extracts (75 and 80% ethanol) and a 40% isopropanol extract induced a concentration-dependent inhibition of all CYP450 isozyme activities, with median inhibitory concentrations (IC50) ranging from 21.9 μg/ml to 65.0 μg/ml. Isolation of the active chemical constituents, showed that the triterpene glycosides were weakly active (IC50 25-100 μM), while fukinolic acid and cimicifugic acids A and B strongly inhibited all CYP isozymes (IC50 1.8-12.6 μM). None of the extracts inhibited the growth of Hep-G2 cells in concentrations up to 50 μg/ml. These data suggest that BCEs are not directly hepatotoxic, but may have the potential to induce herb-drug interactions, which may in turn explain the rare cases of hepatotoxicity observed in women using multiple medications and dietary supplements, including black cohosh.