In the study, published Tuesday by the New England Journal of Medicine, researchers found that the men taking Truvada, a common combination of two antiretroviral drugs, were 44 percent less likely to get infected with the virus that causes AIDS than an equal number taking a placebo.
But when only the men whose blood tests showed that they had taken their pill faithfully every day were considered, the pill was more than 90 percent effective, said Dr. Anthony S. Fauci, head of the infectious diseases division of the National Institutes of Health, which paid for the study along with the Bill and Melinda Gates Foundation.
“That’s huge,” Dr. Fauci said. “That says it all for me.”
The large study, nicknamed iPrEx, included nearly 2,500 men and was coordinated by the Gladstone Institutes of the University of California, San Francisco.
The results are the best news in the AIDS field in years, even better than this summer’s revelation that a vaginal microbicide protected 39 percent of all the women testing it and 54 percent of those who used it faithfully.
Also, Truvada, a combination of tenofovir and emtricitabine that prevents the virus from replicating, is available by prescription in many countries right now, while the microbicide gel is made in only small amounts for clinical trials.
The protection, known as pre-exposure prophylaxis, is also the first new form available to men, especially men who cannot use condoms because they sell sex, are in danger of prison rape, are under pressure from partners or lose their inhibitions when drunk or high.
It “does not involve getting permission from the other partner, and that’s important,” said Phill Wilson, president of the Black AIDS Institute, which focuses on the epidemic among blacks.
Michel Sidibé, the head of the United Nations agency that fights AIDS, called it “a breakthrough that will accelerate the prevention revolution.”
AIDS experts and the researchers issued several caveats about the study’s limitations, emphasizing that it looked only at gay men and Truvada. More studies, now under way, are needed to see whether the results can be duplicated, whether other antiretroviral drugs will work and whether they will protect heterosexual men and women, prostitutes and drug users who share needles.
There is no medical reason to think the pill would not work in other groups, since it attacks the virus in the blood, not in the vaginal wall as a microbicide does. Pre-exposure prophylaxis became possible only in recent years as newer, less-toxic antiretroviral drugs were developed.
Some scientists fear that putting more people on the drugs will speed the evolution of drug-resistant strains, though that did not occur in the study.
Because Truvada is available now, some clinicians already prescribe it for prophylaxis, Dr. Fauci said, but whether doing so becomes official policy will depend on discussions by the Centers for Disease Control and Prevention, the Food and Drug Administration, medical societies and others, which could take months.
Although the C.D.C. would prefer that doctors wait for further studies, more will probably prescribe the drugs now that this study is out, said Dr. Kevin Fenton, chief of the agency’s AIDS division, so the C.D.C. will soon release suggested guidelines.
The agency will suggest that the drug be prescribed only with close medical supervision and used only with other safe-sex practices.
“The results are encouraging, but it’s not time for gay men to throw away their condoms,” Dr. Fenton said.
AIDS advocacy groups were very excited by the results.
“If you comply with it, this works really well,” said Chris Collins, policy director of amfAR, the Foundation for AIDS Research. “This is too big to walk away from.”
Mitchell Warren, executive director of AVAC, an organization that lobbies for AIDS prevention, called the study “a great day for the fight against AIDS” and said gay men and others at risk needed to be consulted on the next steps.
In the study, 2,499 men in six countries — Brazil, Ecuador, Peru, South Africa, Thailand and the United States — were randomly assigned to take either Truvada or a placebo and were followed for up to three years. For ethical reasons, they were also given condoms, treatment for venereal diseases and advice on safe sex. There were 64 infections in the placebo group and 36 in the group that took Truvada, a 44 percent risk reduction.
Two in the Truvada group turned out to have been infected before the study began. When the remaining 34 were tested, only 3 had any drug in their blood — suggesting that the other 31 had not taken their pills.
Different regimens, like taking the pills not daily but only when sex is anticipated, also need testing.
Also, many men in the study failed to take all of their pills, and some clearly lied about it. For example, some who claimed to take them 50 percent or 90 percent of the time had little or no drug in their bloodstreams.
The pills caused no major side effects, though men who began to show signs of liver problems were taken off them quickly. Some men stopped taking the pills because they disliked relatively minor side effects like nausea and headaches. Also, some stopped bothering once they suspected that they might be taking a placebo.
“People have their own reasons,” Mr. Collins said. “People don’t take their Lipitor every day either.”
A major question now is who will pay for the drug.
In the United States, Truvada, made by Gilead Sciences, costs $12,000 to $14,000 a year. In very poor countries, generic versions cost as little as 40 cents a pill.
Globally, only about 5 million of the 33 million people infected with the AIDS virus are on antiretroviral drugs, and in an era of tight foreign-aid budgets, that number is not expected to rise quickly.
Hundreds of millions of Africans, Eastern Europeans and Asians are at risk and could benefit from prophylaxis, but that would cost tens of billions of dollars.
In this country, insurers and Medicare normally pay for the drugs, and the Ryan White Act covers the cost for the poor, but none of these payers yet have policies on supplying the drugs to healthy people.
No participant in the study developed resistance to tenofovir. Three were found to have strains resistant to emtricitabine, but investigators believe that all three were infected before the study began at levels low enough to have been missed by their first H.I.V. tests.
Another concern was that the participants would become so fearless that they would stop using condoms, but the opposite effect was seen — they used condoms more often and had fewer sex partners. But that can also be a result of simply being enrolled in a study and getting a steady diet of advice on safe sex and free condoms, the investigators said.
Other trials of pre-exposure prophylaxis have about 20,000 volunteers enrolled around the world. Their results are expected over the next two years.
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