What Is Drug Resistance?

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What Is Drug Resistance?

This is an adapted selection from Introduction to ART, a booklet published by HIV i-Base, a British treatment activist group, which is also available in a pocket version.

What is drug resistance? Drug resistance occurs when the virus changes its structure in a way that stops a drug from working. These changes are called drug mutations.


What Causes HIV Drug Resistance?

  • The risk of resistance increases when drug levels drop below a minimum active level. This usually only occurs if you miss doses or stop treatment.
  • Resistance only develops if you are on treatment or in the short period after stopping treatment.
  • You can be infected (or reinfected) with drug resistant HIV. About 1 in 10 new infections in the UK have resistance to at least one drug or class of drug.

This is why everyone should have a resistance test when they are diagnosed and before starting ART. But you may need to ask for this test, so it is important to check.


When Does Resistance Occur?

Mutations that cause drug resistance generally only develop on ART when your viral load is detectable. If your viral load is still above 500 copies/mL after 2-3 months, or above 50 copies/mL after 6 months, you can develop resistance.


When Viral Load Increases

If your viral load increases on ART this can also cause drug resistance. Your doctor should look for why this happened. This will involve talking about adherence and side effects.

You might need a drug resistance test or to check your drug levels.


What Happens if My Viral Load Becomes Detectable Again?

If your viral load becomes detectable, the viral load test needs to be repeated within 2-4 weeks of the first blood sample. Often this is a laboratory or test error. Small increases that go back down again are called "blips". The second test will help find out what is happening.


Changing ARVs When Viral Loads Are Low

If the combination is failing it is good to confirm this early. You will get a better response to a second treatment if you change when viral load levels are still low. See the i-Base Guide to Changing Treatment and Drug Resistance for more information.


How Do I Avoid Resistance?

The best way to avoid resistance is to take your drugs every day and on time. Avoiding resistance is more important than increasing your CD4 count because it will let your treatment work for many years. Having an undetectable viral load (less than 50 copies/mL) dramatically reduces the risk of resistance.


Missing Doses?

Drug doses are based on average levels being high enough to be active against HIV for 24 hours. They are also low enough to minimize the risk of side effects.

Some drugs have a higher risk of resistance. If you are having trouble with adherence, your clinic can suggest ART that allows for this.


What Is Cross-Resistance?

Cross-resistance is when a drug mutation against one drug causes other similar drugs to fail, even if you have never taken them before. This is particularly true of drugs in the same class. So if you develop resistance to one NNRTI such as rilpivirine then efavirenz (another NNRTI) is unlikely to work.


Missed or Late Doses

A missed or late dose increases the risk of resistance.

Missing or being late with a drug lets the drug levels fall to a level where resistance can develop. The more often you are late, the greater the chance of resistance.