Safety and efficacy are key issues in antiretroviral therapy (ART) selection. Efavirenz (EFV) is an important component of combination ART in treatment naive individuals. Like many drugs, there are inter-individual differences in the efficacy and tolerability of EFV. The Encore1 study provides an opportunity to examine the pharmacokinetics (PK)(processes by which a drug is absorbed, distributed, metabolized, and eliminated by the body) of EFV in blood samples collected over a 24-hour dosing interval in participants receiving either standard 600 mg or reduced 400 mg dose EFV once daily.

This sub-study will investigate the relationships between dosage, EFV plasma concentrations, toxicity and virological efficacy. EFV concentrations in dried blood spots and matched plasma and will be evaluated to determine the utility of dried blood spot measurements in measuring EFV plasma concentrations. Measurements dried blood spots could potentially be a cheap and easy alternative to measurements in plasma. Dried blood spots can be easily collected from venous blood or fingerprick, do not need plasma separation and potentially need less stringent storage conditions during shipment.

All participants enrolled into the main Encore1 study at participating sub-study sites will be eligible to participate.

Participants must meet the following additional inclusion criteria prior to intensive pharmacokinetic assessment. Inclusion Criteria:

• provide written sub-study consent at or before week 0
• taken randomized study drugs for at least 4 weeks but less than 8 weeks
• taken EFV in the evening for at least 7 days
• taken all EFV doses over the 3 preceding days.