Adherence to an effective antiretroviral (ARV) regimen is the most important determinant of treatment success in patients receiving ARV. It is now well established that virological suppression on highly active antiretroviral therapy (HAART) requires strict adherence to prescribed dosing schedules. It is essential to reach and maintain therapeutic levels of these drugs, and strict adherence is particularly crucial for preventing the development of drug-resistant viral strains. If therapeutic drugs levels are suboptimal and the virus is able to replicate, this has important individual and public health implications.