The nature of many viruses, including HIV, is to infect a host organism (i.e., a cell; often having a detrimental effect), create copies of itself—replicate, duplicate, multiply—and spread to other hosts. When a virus multiplies, changes in its form (called mutations) result in variations of the virus.

According to the National Institutes of Health (NIH) of the U.S. Department of Health & Human Services, variations can lead to drug-resistant strains of the virus. When drug resistance occurs, antiretroviral medications that should help prevent HIV from multiplying are ineffective. A person may be infected with a drug-resistant strain of HIV, or may develop resistance after starting antiretroviral therapy.

Drug-resistance testing (a blood test) is used immediately after HIV diagnosis and before antiretroviral treatment to determine if the virus is drug-resistant. The results are used to develop a treatment plan—that is, help the health care team decide which ART drugs to include. The test also is performed during HIV treatment, especially if viral load testing indicates the virus is not being controlled. If drug resistance has developed, a new treatment regimen is started.

Poor medication adherence—failing to take all HIV medicines exactly as prescribed—increases the risk for drug resistance. Skipping medications or taking them incorrectly allows the virus to multiply, increasing the chance that the HIV will mutate. When this occurs, one or more drugs in an HIV regimen may become ineffective.

Cross Resistance

Resistance to one HIV medication may lead to resistance to other HIV drugs in the same class. This is called cross resistance. When this occurs, the virus may become resistant to medications the person has never taken, limiting the number of effective drugs available to him/her.

Adherence to an HIV regimen is extremely important to prevent drug resistance and cross resistance. Be sure to keep all medical appointments and talk to your health care team if you are having difficulty with adherence.

Source: National Institutes of Health (NIH)

Publication Review By: the Editorial Staff at

Published: 29 Nov 2015

Last Modified: 31 Mar 2017