Resistance to anti-HIV drugs
If you miss doses of your anti-HIV drugs then the levels of these drugs in your blood will fall. This will allow your HIV to become more active, increasing the speed in which it replicates, or makes copies of itself.

HIV often makes small errors when making these copies. Some of these errors, or mutations, can make the virus resistant to one or more of the anti-HIV drugs present in your blood. When this happens, these mutations are referred to as ‘resistance mutations’. Therefore, missing doses of your anti-HIV drugs gives your HIV the opportunity to develop resistance mutations which could render one or more of the drugs you are taking ineffective. The more doses you miss, the more likely this will occur.

If you do develop resistance to one or more of your anti-HIV drugs, you would first notice a rise in your viral load test results. Your doctor would probably want to test you again to see if the rise is a continuing trend, as random blips in your viral load can happen from time to time. If the trend continues, your doctor would probably want to perform a resistance test before recommending a change in the combination you are on to a combination consisting of drugs that your HIV is not resistant to.

Developing resistance to some anti-HIV drugs can not only render those specific drugs ineffective, but also other drugs in the same class that you aren’t even taking. This is known as cross-resistance. For example, developing resistance to the NNRTI efavirenz would usually mean that you would also then be resistant to the other currently available drugs in that class. In this case the whole NNRTI class would be of no use any more, and your doctor would recommend changing to a regimen that contains only drugs from other classes.

We talk more about changing your combination of anti-HIV drugs in the section on Changing Treatment.

Even if you don’t miss doses and take your drugs on time all the time, there is still the chance that some drug resistance will eventually occur. Having said this, after years of good adherence many men are still doing very well on their first combination of anti-HIV drugs.

Multiple drug resistance
Multiple drug resistance is when your HIV becomes resistant to drugs from two or more classes of anti-HIV drugs. This can become a real problem as effective drug options for people with multiple drug resistance can be extremely limited.

Continued poor adherence to one combination of anti-HIV drugs after another would cause your HIV to develop multiple drug resistance. This is why adherence is so important in successfully controlling HIV.

Drug resistant HIV is usually much less able to do the damage to your immune system than HIV with no resistance, known as wild type HIV. This is because the mutations HIV develops to make it resistant to anti-HIV drugs often reduce its ability to infect CD4 cells and make copies of itself. The strength of your HIV virus is referred to as its viral fitness. HIV with many mutations will often not be as fit as the original wild type virus.

Treating people who have failed on two combinations of drugs or more is known as salvage therapy. This often involves many different drugs, including some that the person’s HIV is partially resistant to, and also also newer drugs such as entry or integrase inhibitors.

You can read more about salvage therapy in the section on Changing Treatment.

There are new drugs that are being developed that are effective against HIV even if it is resistant to most of the other drugs in that class. These new drugs will be vital in controlling HIV in those people who are failing on combinations containing the currently available anti-HIV drugs.

You can read more about the new anti-HIV drugs being developed in the section on the Future of HIV Treatment.

Infection with drug resistant HIV 
If your HIV has developed drug resistance and you pass it on to someone else, they will be infected with this drug resistant strain, even if they have never taken any anti-HIV drugs. This means that even before starting HIV treatment, their treatment options are limited. How limited they are depends on how resistant the strain of HIV they were infected with is.

Discovering what anti-HIV drugs someone’s HIV is resistant to is done by performing a resistance test. This test should always be done before someone starts treatment, or needs to change treatment because their current combination of drugs is failing.