When someone has drug-resistant HIV, this means that the HIV in their body has some mutations. Because of the mutations, some anti-HIV drugs may not work well. While a mutation may render one anti-HIV drug ineffective, it will not affect other drugs, which remain effective. Drug resistance can develop when a person has HIV and takes an inadequate amount of anti-HIV drugs (for example, two drugs rather than a three-drug combination, or misses doses).

For people who take PrEP and do not acquire HIV, there is no possibility whatsoever of having drug-resistant HIV.

In theory, if people are not fully adherent to PrEP, become HIV positive, are not diagnosed and carry on taking PrEP, then drug resistance could develop. But only a handful of cases like this have been documented in PrEP studies.

What has been seen are some cases of individuals who started taking PrEP when they were already in the very early stages of HIV infection. Although an HIV test before beginning PrEP is standard practice, a test may miss recent infection due to each test’s ‘window period’. Nonetheless, not all people who start PrEP while they are in very early infection develop drug resistance and most who do develop resistance to emtricitabine, not to tenofovir. To avoid this occurring, people beginning PrEP should be carefully checked for possible symptoms of seroconversion (e.g. fever, rash).

Strains of HIV which are resistant to some specific anti-HIV drugs can be transmitted during sex. This raises the question of whether PrEP would work if somebody taking it were exposed to drug-resistant HIV.

For the moment, this is a theoretical concern. While there is a strain of HIV (known as K65R) which is resistant to tenofovir and could make PrEP ineffective, only 1 in 1000 transmissions of HIV involve virus with this mutation. No cases of PrEP failure due to drug resistance have been documented.