Even when you take ARVs correctly the virus can change it's form. Then your doctor needs to change the combination of medicines you first started taking (first-line regimen) to a different combination of ARVs (second-line regimen). If you take your medicines incorrectly you may have to start your second line regimen sooner. It's better to make your first-line regimen last as long as possible.

Attribution: Straight Talk Foundation & Marie Stopes