Scientists have an exciting breakthrough in the fight against AIDS. A pill already used to treat HIV infection turns out to be a powerful weapon in protecting healthy gay men from catching the virus, a global study found.

Daily doses of Truvada cut the risk of infection by 44 percent when given with condoms, counseling and other prevention services. Men who took their pills most faithfully had even more protection, up to 73 percent.

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As a practical matter, price could limit use. The pills cost from $5,000 to $14,000 a year in the United States, but only 39 cents a day in some poor countries where they are sold in generic form.

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The company donated Truvada for the study, which involved about 2,500 men at high risk of HIV infection in Peru, Ecuador, Brazil, South Africa, Thailand and the United States (San Francisco and Boston). The foreign sites were chosen because of high rates of HIV infection and diverse populations.

More than 40 percent of participants had taken money for sex at least once. At the start of the study, they had 18 partners on average; that dropped to around 6 by the end.

The men were given either Truvada or dummy pills. All had monthly visits to get HIV testing, more pills and counseling. Every six months, they were tested for other sexually spread diseases and treated as needed.

After a median followup of just over a year, there were 64 HIV infections among the 1,248 men on dummy pills, and only 36 among the 1,251 on Truvada.

Among men who took their pills at least half the time, determined through interviews and pill counts, the risk of infection fell by 50 percent. For those who took pills on 90 percent or more days, risk fell 73 percent. Tests of drug levels in the blood confirmed that more consistent pill-taking gave better protection.

The treatment was safe. Side effects were similar in both groups except for nausea, which was more common in the drug group for the first month but not after that. Unintended weight loss also was more common in the drug group, but it occurred in very few. Further study is needed on possible long-term risks.

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