Pharmacogenetics may also play an important role in antiretroviral pharmacokinetics in the older population. Several polymorphisms have been shown to be associated with adverse effects to nucleoside reverse transcriptase inhibitors (NRTI) (Tozzi 2010). The very strong association between the abacavir hypersensitivity reaction and Human Leukocyte Antigen (HLA) type B*5701 is an excellent example that has changed clinical practice (Mallal 2002). Nonnucleoside reverse transcriptase inhibitor (NNRTI) hepatotoxicity also appears to be associated with a Multi Drug Resistance 1 (MDR1)gene polymorphism as the 3435 CT genotype confers reduced risk (Haas 2006). Efavirenz is primarily hepatically metabolized. Clearance of efavirenz occurs predominantly via Cytochrome 2B6 (CYP2B6) to an 8-hydroxy-efavirenz (8-OH-Efavirenz); additionally there is a subsequent pathway via cytochrome CYP2A6 to a7-OH-Efavirenz metabolite (Avery 2012, di Iulio 2009, Markwalder 2001).CYP2B6 polymorphisms have been observed inpatients with increased frequency of efavirenz-related side effects, and are associated with drug discontinuation (Haas 2004, Ribaudo 2006). In addition to CYP2B6, early treatment discontinuation of efavirenz was also recently associated with a polymorphism in the constitutive androstane receptor (Wyen 2011). Within this study we will correlate efavirenz plasma metabolite concentrations with pharmacogenetic data on CYP2B6 and CYP2A6 status in order to define the role of these metabolizing enzymes in efavirenz concentrations in older HIV-infected individuals. Our goal will be to assess whether CYP2B6 and CYP2A6 status exacerbate efavirenz-related side effects in this aging population.