The nonnucleoside reverse transcriptase inhibitor efavirenz is metabolized by the hepatic cytochrome P450 isoenzyme 2B6.1 Homozygotes for a single polymorphism at position 516 in the CYP2B6 enzyme (G > T) have an approximately 3-fold higher area under the curve of efavirenz plasma concentrations.2 Central nervous system toxicity is associated with elevated efavirenz plasma concentrations, and lack of response is associated with low efavirenz plasma concentrations.3 The 516T polymorphism represents alleles *6, *7, and *9.4 The prevalence seems to differ by race with the highest rates found to date in populations of African origin.5 Efavirenz is commonly used in southern Africa. Although slow metabolizing genotypes might have an important impact on human immunodeficiency virus (HIV) treatment, the prevalence in this setting is currently unknown. We conducted a cross-sectional study of a convenience sample of HIV-infected Botswana citizens (Batswana) under care at the Infectious Diseases Care Clinic of the Princess Marina Hospital in Gaborone, Botswana. All individuals of African origin attending clinical visits over 1 week in July 2007 were invited to participate. Buccal mucosa was obtained using sterile brushes and stored at −80°C. DNA was extracted using the Qiagen protocol (Qiagen, Valencia, CA). CYP2B6 genotyp

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