This is called drug resistance, and it's one of the most common reasons why HIV therapy fails. Drug-resistant HIV then needs higher levels of the same drugs to stop it from reproducing. In general, when four or more times as much drug is needed to suppress the virus in a test tube, the virus is considered resistant to that drug. Increasing doses of drugs to overcome resistance is not possible because higher doses lead to increased risk of side effects. So when resistance occurs, people often need to change to a new anti-HIV drug combination.
Resistance is usually due to changes in viral genes, called mutations.
Because HIV mutates easily and reproduces very rapidly, a person may have many different HIV strains in his or her body. Today's drugs are not completely able to stop HIV from reproducing. If a resistant virus develops, it can grow in the presence of the drug and viral load subsequently starts to rise. Drug-resistant virus will grow more quickly and become the most prominent virus in a person's blood. Lab tests can show whether a person's virus is likely to be suppressed by each anti-HIV drug.