Abstract
Efavirenz is in the non-nucleoside reverse transcriptase inhibitor category of HIV antiretroviral medicines. It is an in vivo inducer of the CYP3A4 isoenzyme within the cytochrome P450 (CYP450) system, and an in vitro inhibitor of the system’s CYP2C9/2C19, 3A4 and 2B6 isoenzymes; as a result, concentrations of psychotropic drugs can be increased or decreased depending on the specific enzyme pathway involved in their metabolism. CYP3A4 is responsible for metabolizing many benzodiazepines and other psychotropics, as well as selective serotonin reuptake inhibitors and tricyclic antidepressants. As an inducer of CYP3A4, efavirenz can increase the rate at which these agents are metabolized, resulting in administered psychotropic drug levels that are below their therapeutic thresholds. Conversely, efavirenz is an inhibitor of CYP2B6, which metabolizes agents such as bupropion; consequently, bupropion levels in the blood can increase. Given the existing conflicting data, the clinician may find it impractical to use an evidence-based approach when concomitantly prescribing efavirenz and psychotropic drugs to their HIV patients. Instead, it may be preferable to use a more pragmatic approach that applies knowledge of the most current pharmacological and pharmacokinetic data for psychotropics and non-nucleoside reverse transcriptase inhibitors, which may help better predict their potential interactions.