As more people start antiretroviral treatment (ART), HIV drug resistance is becoming more common. The level of drug resistance varies according to several factors including the class of drug. Resistance to NNRTIs (such as efavrienz and nevirapine) is more common than resistance to newer drugs like dolutegravir.

The World Health Organization recommends that countries should urgently change their first-line HIV treatment to a dolutegravir-containing regimen if 10% of their population has pre-treatment drug resistance to NNRTIs.

Pre-treatment drug resistance is also more common among infants who may have exposure to lower levels of antiretroviral treatments druring pregnancy and while breastfeeding.

Yes - although drug resistance most commonly develops in people who have started treatment and had problems with adherence, it's possible to be infected with HIV that has already become resistant. This is known as transmitted drug resistance and happens when the person who passes HIV on to you has drug resistance. To prevent drug resistant HIV from being passed on, you just need to use the normal prevention methods including PrEP, condoms and being undetectable. This is one of the reasons why it can be advised that two people with HIV continue to use condoms, so that they avoid transmitting different types of drug resistance to each other.

Cross-resistance is where being resistant to one drug means that you can no longer use other antiretroviral medicines that are similar. For example, if you become resistant to efavirenz it can rule out other drugs that are too similar as they will no longer be able to control your HIV either.

Another way of getting HIV drug resistance is if you have taken a form of ART previously but then stopped, for example if you took ART during pregnancy and breastfeeding for PMTCT but then didn't continue, or if you took PrEP or PEP while being positive.

Because HIV drug resistance is linked to adherence it becomes a bigger problem in places where people face more barriers to getting treatment and care. Risk factors are generally divided into three categories: patient factors, programme factors and treatment regime factors. We outline some of the main examples of each below.

Patient factors: HIV drug resistance becomes more of a problem in places where individuals can't access the support they need to adhere to treatment. People are less likely to adhere to treatment if they have a poor understanding of HIV or the benefits of treatment. Where people have other health problems such as depression or substance abuse, it can make adherence more difficult and increase their risk of developing drug resistance.

Programme factors: A good example of a programme factor is drug stock-outs. Drug stock-outs are usually the result of poor planning, procurement and supply chain management (these are the steps required for clinics to buy and get the right antiretroviral treatment). If clinics are not able to get all the drugs that they need, people will miss doses of their treatment, making them more likely to become resistant. Similarly, where programmes don't provide viral load testing, HIV drug resistance is a greater risk as it's harder for doctors to know when treatment failure has occured and a patient needs to switch to a different treatment.

Treatment regime factors: If someone's regime is more complicated - for example they are taking a high number of pills at different times of the day - they are at greater risk of developing drug resistance. This is simply because it's hard to keep track of what they are supposed to take. Some drugs can have a 'higher barrier to resistance'. For example, because of how dolutegravir works, HIV is less likely to become resistant to it making it a better option especially for people who have difficulty adhering.