(HealthDay News) — In a letter to the editor published in the Feb. 2 issue of the New England Journal of Medicine, authors present the case of a Canadian man infected with HIV despite long-term adherence to pre-exposure prophylaxis (PrEP).

Based on a genetic analysis of the virus, it was determined that the 43-year-old Toronto resident was infected with a strain of HIV that had become resistant to emtricitabine and tenofovir disoproxil fumarate (Truvada), report author David Knox, M.D., of the Maple Leaf Medical Clinic in Toronto, told HealthDay.

The patient began taking oral Truvada in April 2013. Pharmacy records show he had been taking the drug as prescribed, Knox and colleagues noted. But after two years of successful PrEP, a screening test revealed that the man had contracted HIV. The man was not using condoms during sex, Knox noted, which is recommended for complete protection against HIV, even for patients taking PrEP. However, the man also did not have any other sexually transmitted diseases.

“We should be monitoring the rates of these mutations that are occurring in individuals with HIV, and we should be vigilant for unique cases in PrEP users,” Knox said. “There’s tens of thousands of people who are believed to be on HIV PrEP, and there have been only two documented cases of people who were infected with drug-resistant strains of HIV. I wouldn’t use this case as a reason to stop taking PrEP or to dissuade anybody from initiating PrEP if they are at high risk for HIV.”

Several authors disclosed financial ties to pharmaceutical companies, including Gilead, the manufacturer of Truvada.

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