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. 

  • Resistance only develops if you are on treatment or in the short period after stopping treatment.
  • 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. (See A missed or late dose increases the risk of resistance).
  • You can be infected (or reinfected) with drug resistant HIV.

About 1 in 12 new infections in the UK have resistance to at least one drug or class of drug.

This is why in the UK 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.

ResistanceWhen 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.

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. 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.

Last updated: 1 October 2019.