Some of them experience side effects. Many are minor, but some can be debilitating. Often, these side effects go away after a few weeks. Some can last longer, but won't stop you from your daily activities. When an antiretroviral drug begins to lose its effect, it is usually because the virus has developed resistance to the drug.
Resistance occurs when certain changes in the virus make it less sensitive to a drug.
HIV is particularly good at developing drug-resistant mutations. This is a major reason why the virus has been so difficult to eliminate. Drug resistance can develop to any antiretroviral drug, especially when the drug is given alone instead of combined with other antiretrovirals. Resistance also occurs more frequently if a drug is not consistently taken according to the treatment schedule. Using a combination of antiretroviral drugs has many advantages.