As HIV multiplies in the body, the virus sometimes mutates (changes form) and produces variations of itself. Variations of HIV that develop while a person is taking HIV medication can lead to drug-resistant strains of HIV, meaning the HIV medications that previously controlled a person's HIV are not effective against the new, drug-resistant strain of HIV. In other words, the HIV medications can't prevent the drug-resistant HIV from multiplying. As a result of drug resistance, one or more HIV medications in a person's HIV regimen may not longer be effective.
A person can be initially infected with drug-resistant HIV or develop drug-resistant HIV after starting HIV medications. Drug-resistance testing is used to identify which HIV medications (if any) won't be effective against an individual's HIV. Drug-resistance testing results help determine which HIV medicines to include in an HIV treatment regimen.