PrEP is not "an infallible magic bullet," says one researcher.

A man has contracted HIV while on a daily regimen of Truvada, reports POZ magazine.

The individual, a 43-year-old gay man, was exposed to a drug-resistant strain of the virus, and is believed to be the first documented case of someone contracting HIV while taking Truvada regularly.

If taken properly PrEP is 92%–99% effective, though strains resistant to both of Truvada’s two components, tenofovir and emtricitabine, have started to emerge.

Pharmacy records indicate the man filled his prescription regularly, but recently tested positive after 24 months on the drug.

The incident demonstrates that PrEP is not “an infallible magic bullet,” says researcher Richard Harrigan.

“It is an example that demonstrates that PrEP 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.”