Recent concerns over the effectiveness of PrEP have emerged after the sixth confirmed failure of the drug was reported earlier this year, when a man in San Fransisco contracted HIV despite consistently taking the HIV prevention medicine. Experts are assuring people on PrEP that it is still the most effective way to avoid contracting HIV.

PrEP, or pre-exposure prophylaxis, involves taking a Truvada pill once daily and is designed to prevent contraction of the HIV virus.

The San Fransisco man’s blood and hair samples indicated that he adhered to regularly taking the HIV prevention medicine over the past three months, but was infected with a strain of HIV that was resistant to emtricitabine, but still susceptible to tenofovir.

Despite recent concerns, HIV Ireland says that PrEP is “proven to be safe and very effective to stop HIV from establishing itself inside the body. Taking PrEP once every day reduces the risk of getting HIV from sex by more than 90%, and by more than 70% among people who inject drugs. PrEP stops HIV from taking hold and spreading throughout your body.”

Medical director at the San Francisco City Clinic, where this recent failure was recorded, Dr Stephanie Cohen told NBC that PrEP is working as expected, but added that “nothing is 100% effective, unfortunately.”

Dr Stephanie Cohen’s clinic operates San Francisco’s largest PrEP program and is at the forefront of a nationwide effort to promote and distribute PrEP to at-risk people.

Including this case in San Fransisco, four of the six confirmed PrEP failures from around the world since 2012 have involved rare, resistant HIV strains.

A man who had been taking PrEP was infected with an HIV strain that was resistant to both tenofovir and emtricitabine in Toronto in 2016, while only one case of PrEP failure involved an HIV strain that was not resistant to any drugs.

Experts say that in all cases of PrEP failures, patients quickly became HIV-undetectable after switching from Truvada to a new medication.