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 (click on it to see a bigger version).

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. However, newer anti-HIV drugs provide treatment options for people who have developed resistance to other drugs and resistance does not need to be a reason for someone to become seriously ill because of HIV.

Once resistance to one anti-HIV drug has developed, this may mean that your HIV is also resistant to other, similar anti-HIV drugs (that is, drugs from the same ‘class’) you haven’t yet taken. This is called cross-resistance.

However, cross-resistance isn’t inevitable if you develop resistance to one drug. The use of resistance tests will help you and your HIV doctor select the anti-HIV drugs that are likely to be most effective against your virus if resistance does develop. The risk of cross-resistance is lower if your treatment combination is changed promptly and also for many newer anti-HIV drugs. For these reasons, an undetectable viral load is now a realistic aim for most people.