Image Increasing use of Truvada was linked to declining rates of H.I.V. infection, researchers found, but also to decreased use of condoms. Credit... Thor Swift for The New York Times

The rollout of a drug that prevents H.I.V. infection was followed by a reduction in condom use among gay and bisexual men in Australia, according to a study published in the journal Lancet H.I.V.

But so effective was the drug that H.I.V. infection rates in the study region declined anyway, the researchers concluded.

During the rapid distribution of a drug that prevents infection — a strategy called pre-exposure prophylaxis, or PrEP — among gay and bisexual men in Australia, researchers found that unprotected sex increased even among those not on PrEP, suggesting that perceptions of risk had declined in communities where the drug was widely available.

“Safe sex is basically transformed by PrEP,” said Martin Holt, a researcher at University of New South Wales in Sydney, who led the study.