The reason why patients are failing 2nd line treatment is not exactly known. Failure has been previously attributed to poor adherence. However, some literature shows that some patients on boosted PIs achieve and maintain viral suppression despite suboptimal adherence (adherence of 80- 95%). Viral factors, like drug resistance, are also implicated in treatment failure. However, boosted PIs have high genetic barrier to clinically significant mutations. Therefore, a virus would have to harbour multiple PI mutations for the virus to have reduced susceptibility to boosted PI regimens. Pharmacological factors such as suboptimal dosing, impaired absorption and drug interactions may also be responsible for treatment failure.