As has already been mentioned, one of the possible consequences of not taking your HIV treatment properly is that your HIV will develop resistance to anti-HIV drugs. This section of the booklet provides information on how resistance can develop; how to reduce the risk of resistance; cross-resistance; resistance tests; and the transmission of resistant virus.

HIV reproduces itself very quickly, making billions of new viruses every day. Because the virus often makes mistakes when copying itself, each new generation differs slightly from the one before. These tiny differences are called mutations.

Some mutations occur in the parts of HIV which are targeted by anti-HIV drugs. This can result in strains of HIV developing that are less easily controlled by the drugs. These HIV strains are called drug-resistant.

Drug-resistant HIV strains vary – some may be highly resistant to anti-HIV drugs while others may be less so. When an anti-HIV drug is started, HIV that is fully susceptible to that drug disappears rapidly and drug-resistant viruses remain. If the HIV replication is not fully suppressed, these resistant viruses can continue to reproduce themselves despite the drug's presence. The diagram below shows how this works.

Resistance is an important reason why HIV treatment can fail. Viral load, which should drop when you start a new drug combination, will increase again if drug-resistant HIV is able to emerge.