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.
“It’s great that these guys are feeling optimistic about avoiding H.I.V., but it has disrupted prevention methods at a community level.”
The daily pill is highly effective in combating H.I.V. infection and is recommended for at-risk populations, such as gay men and intravenous drug users, according to the World Health Organization.
The cross-sectional survey included almost 17,000 men in Victoria and New South Wales who said they had recently had casual sex with a male partner.
From 2013 to 2017, the proportion of H.I.V.-negative men using PrEP increased to 24 percent from 2 percent. During that time, the proportion of respondents with casual partners who said they consistently used condoms dropped to 31 percent from 46 percent.
Strikingly, unprotected sex among men who did not take PrEP increased 9 percent.
“Rightly or wrongly, there is a sense among people that men can have safer sex without condoms for the first time,” said Mitchell Warren, the executive director of AVAC, which promotes H.I.V. prevention worldwide. “We need a nuanced conversation about the patterns of choices.”
One concern is that PrEP does not protect against other sexually transmitted infections. As use of the pill spreads, some clinicians are detecting more cases of diseases such as gonorrhea, as well as drug resistance to them, according to Dr. Paul Volberding, the director of the AIDS Research Institute at the University of California, San Francisco.
It is not clear whether the higher case counts are a result of decreased condom use or merely better surveillance.
“Does this mean that pushing PrEP is a bad thing? Of course not. It is preventing H.I.V., which is what it was designed to do,” Dr. Volberding said. “But the challenges posed are no minor issue, and we need to take them seriously.”
Dr. Holt said he was uncertain how decreased condom usage will affect rates of H.I.V. infection in the long term, but that so-called herd immunity — the protective effects of a growing population of men who use PrEP and do not transmit H.I.V. — could at least partially counterbalance the effects of unsafe behaviors.