Resistance is the main reason for treatment  failure: it can make your medication less effective  and sometimes stops medicines from working at all.
A high level of adherence can reduce the risk 
of resistance occurring. Likewise, a low level of adherence will increase the likelihood of resistance developing. 

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To understand resistance you need to know that HIV makes billions of new copies every day. Making this many copies can lead to some mistakes; these are copies that are slightly different and they are known as mutations. These mutations can alter the part of the virus targeted by antiretrovirals (HIV treatments), which may mean the treatment is less effective. When this happens, it is known as drug-resistant HIV.

There are varying degrees of drug resistance. Some strains of HIV can be completely resistant to a drug, yet other strains may only be partially resistant. Resistance will impact how effective your medication is and it can lead to treatment failure.

In the early days of HIV medication, few antiretrovirals existed, so drug resistance greatly limited your treatment options. There are now more than 25 licensed antiretrovirals, but drug resistance remains a problem because some strains of HIV can be resistant to more than one drug.

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Development of resistance

 

A low level of adherence gives HIV a chance to make more copies of itself (known as replication). Missing doses, not taking the right dose or not taking a dose in the right way can affect the level of drug in your body. If this level is too low (known as sub-optimal) the drug will not work as effectively as it could. This gives HIV and its mutations an opportunity to replicate.
 

Antiretrovirals are most effective against standard HIV 
(known as wild type) but they are not so effective against mutations. Although some mutations do not harm the effectiveness of your treatment, many of them can affect how well your treatment works. Since the mutated strains of the virus can be resistant to one or more drugs, the greater the number of mutated versions of the virus that are produced, the less effective your treatment will become.
If the resistant mutations keep replicating, they can become the most common type of HIV in your body, so you could have more mutations than wild type. It is unlikely that your medication will work at all if this happens.
When drug resistance occurs, your doctor will probably suggest changing one or more of the drugs in your treatment regimen.

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Testing for resistance

 

A blood sample can be tested to check for drug resistance. These tests can detect exactly what type of mutations you have and whether your treatment will stop working effectively.

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Treatments stop working when specific mutations are present; the type of mutation that will stop a treatment from working may be different for each medication. Some mutations do not harm the effectiveness of your treatment, while others can eventually lead to resistance to a specific drug. Some mutations can cause resistance to several drugs, known as cross-resistance. This can include drugs you have never been prescribed, limiting future treatment options. Antiretrovirals are divided up into drug classes and it is possible to become resistant to most or all of the drugs in a given class.
There are now many different treatment options available and some of these drugs have better resistance profiles than others. This means they have a greater chance of being effective even when mutations are present. Sometimes it takes a combination of three or more specific mutations to be present before they stop working.
Resistance testing is often used for people who are about to  start or change treatment, for pregnant women and for anyone  whose viral load is still detectable while taking medication.

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