Despite taking an anti-HIV pill consistently for 24 months, a gay man in Toronto, Canada was diagnosed with the infection, making it the first time that the medication "failed."

The physician from Maple Leaf Medical Clinic who treated the man presented the case last week at the Conference on Retroviruses and Opportunistic Infections (CROI) in Boston.

The man consistently took pre-exposure prophylaxis (PrEP) for two years. PrEP is a cocktail treatment consisting of two HIV drugs -- tenofovir and emtricitabine -- which is sold under Truvada.

The treatment is especially designed for people who are not diagnosed with HIV but are at great risk of contracting the virus. It requires taking a antiretroviral pill every day to prevent the infection.

Past research has shown that the Truvada pill is highly effective. The Centers for Disease Control and Prevention (CDC) said that with daily use, the risk of acquiring HIV is lowered at 92 percent. But it supposedly did not work on the patient.

"To our knowledge, this is the first reported case of breakthrough HIV infection with evidence of long-term adherence to [Truvada]," said HIV specialist Dr. David Knox, the lead author of the case.

Knox explained during his presentation that the 43-year-old patient was a regular at the Maple Leaf Medical Clinic who habitually got tested for HIV. The man began the PrEP regimen in April 2013 following a recommendation.

At the time, the man's HIV-positive partner was on antiretroviral therapy with "untraceable" levels. The 43-year-old patient himself reported several acts of intercourse without condoms for protection before he tested positive.

PrEP is known to be ineffective when taken sporadically. Although the patient was "adamant" about his adherence to the regimen, dried blood spot analysis and pharmacy records indicated consistency in the previous one to two months, Knox said.

With that, Knox said the failure of the PrEP in this case was possibly due to a rare transmission of an HIV 1 strain that is resistant to both tenofovir and emtricitabine.

What does this mean for people who are taking the drug? Gawker pointed out that it doesn't mean the drug does not work. It just means that the PrEP is not 100 percent effective, especially if taken inconsistently. What's more, even though a person is consistently taking the drug, it is still important to use protection during intercourse.

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