Efavirenz is an essential component of HIV treatment in children aged 3 years or older on anti-tuberculosis (anti-TB) treatment. However, the appropriate efavirenz dose during anti-TB treatment remains unclear. Rifampin (an anti-TB drug) increases the activity of the drug metabolizing enzymes that breakdown efavirenz, which may lead to low blood levels of efavirenz and treatment failure during cotreatment. The drug-to-drug interactions between the HIV and anti-TB drugs also vary between individuals based on genetic factors. This study will investigate the effects of anti-TB treatment, as well as drug-gene interactions on the blood concentrations of efavirenz in children with HIV and TB infections. Such data could enhance optimization of efavirenz dosage or selection of alternate regimens in some children.