A man taking the HIV-prevention pill Truvada has contracted HIV, marking the first reported infection of someone regularly taking the drug, according to researchers. The report, from the Maple Leaf Medical Clinic in Toronto, says that a 43-year-old man who had sex with other men was infected after taking Truvada for 24 months. The HIV health site Poz reports that he's believed to have encountered a rare strain of the virus that's resistant to the two medicines that make up Truvada. The virus is believed to have been drug-resistant when it was contracted, rather than evolving resistance after transmission.

Truvada is supposed to reduce HIV transmission rates by more than 90 percent; Poz cites current research as putting it above 99 percent when taken four times a week. So news of its failure is a surprise. It doesn't mean the drug is ineffective, however. “I certainly don't think that this is a situation which calls for panic," says Richard Harrigan, an HIV researcher who worked on the study, according to Poz. "It is an example that demonstrates that PrEP [Pre-Exposure Prophylaxis, the use of drugs to prevent infection] can sometimes be ineffective in the face of drug resistant virus, in the same way that treatment itself can sometimes be ineffective in the face of drug resistant virus.”