CYP2B6 (Cytochrome P450 Family 2 Subfamily B Member 6): CYP2B6 is an enzyme that plays a role in the metabolism of various drugs, including bupropion, efavirenz, methadone, and certain anesthetics. Genetic variations in CYP2B6 affect enzyme activity, influencing drug breakdown, effectiveness, and the risk of side effects. Testing CYP2B6 can help optimize medication dosing and reduce the likelihood of adverse reactions.