CYP2B6*18 (c. 983T>C, p.I328T) – No function: CYP2B6*18 is assigned no function (note that the formerly assigned CYP2B6*16 allele is now a suballele of CYP2B6*18). Several clinical studies indicate that CYP2B6*18 is strongly associated with higher plasma efavirenz exposure in patients with HIV (18281305, 23080225, 26655325). Studies evaluating efavirenz content in hair samples among patients with HIV taking efavirenz for more than three months found that patients with the CYP2B6*18 allele had nearly double efavirenz exposure compared to the *1 reference (26655325, 34115651). Another study evaluating plasma efavirenz concentrations in patients with HIV found three-fold higher efavirenz concentrations in patients carrying CYP2B6*18 compared to the *1 reference (16495778). Furthermore, CYP2B6*18 is associated with significantly higher exposure of nevirapine in plasma (18281305, 25878720) and hair (34115651) and lower exposure of hydroxybupropion after bupropion administration (23344581). Lower clearance of methadone compared to the *1 reference was found in a CYP2B6*18 carrier (35538637). Several in vitro studies found that CYP2B6*18 exhibited undetectable (or markedly decreased) protein expression and/or activity towards different probe substrates including efavirenz, bupropion, ketamine, and methadone, in several cDNA expression systems (16272958, 16495778, 20517174, 21746968, 25897175, 30085944, 30201214, 32238417, 35538637). Therefore, consensus among experts was no function based on strong evidence.