For the first time ever documented, a person taking Truvada — the once-a-day pill that prevents HIV infection in what’s known as pre-exposure prophylaxis, or PrEP — has contracted a drug-resistant strain of the virus.

The case was presented on Thursday at the Conference on Retroviruses and Opportunistic Infections (CROI) in Boston.

The patient was a 43-year-old gay man from Toronto who had been on the daily drug regimen for 24 months with no previous issues. The study researchers noted that, according to pharmacy records as well as blood tests, the man had been taking the drug properly around the time that he contracted the virus.

In general, Truvada is extremely effective in preventing HIV in those who take it every day. But the scientists concluded that there is an important, though very rare exception: someone who is exposed to a virus strain that is resistant to both drugs included in Truvada, tenofovir and emtricitabine.

The Toronto man had sex with someone who carried a virus that was resistant to both of the drugs in Truvada, as well as others.

“I think the evidence that we have says that in general PrEP is working quite well,” Richard Harrigan, director of research labs at the British Columbia Centre for Excellence in HIV and one of the study researchers, told BuzzFeed News. “But there are always exceptions, and we have to keep that in mind.”

The problem arises because we use the same drugs for prevention that we use for treatment, Harrigan said.

“In the ideal world, there wouldn’t be any crossover between the drugs we use, but at this point we just don’t have that many things that have been shown to work,” he said.

The man, who is now HIV-positive, is on antiretroviral therapy and has his infection under control, Harrigan said.

Harrigan also cautioned that exposure to a strain of HIV that’s resistant to just one of the drugs in Truvada might decrease the effectiveness of the pills. According to research done by his group, roughly 10% of the HIV-infected British Columbia population has emtricitabine resistance, and about 1% to 2% have tenofovir resistance.

“The issue of resistance is one that’s been discussed at length with respect to PrEP,” Mitchell Warren, executive director of AVAC, a global HIV-prevention advocacy group in New York City, told BuzzFeed News.

But he noted that, for example, none of the 1,400 people taking PrEP in an ongoing study led by Kaiser Permanente San Francisco have contracted the virus.

“I think it’s certainly important evidence that needs to be understood,” Warren said. “I don’t think it undercuts the important role PrEP can play.”