ATLANTA — An AIDS drug already shown to help prevent spread of the virus in gay men also works for heterosexual men and women, two studies in Africa found. Experts called it a breakthrough for the continent that has suffered most from AIDS.

“These studies could help us to reach the tipping point in the HIV epidemic,” Michael Sidibe, executive director of the United Nations AIDS program, said in a statement as the study results were announced.

“This is really a game-changer,” said Dr. Jared Baeten, a University of Washington researcher who was a leader of one of the studies.

The prevention drug is Truvada, a pill already on pharmacy shelves to treat people with HIV. It's made by Gilead Sciences Inc. of Foster City, Calif. Another Gilead drug, Viread, was also used in one of the two African studies. Earlier research with Truvada found it prevented spread of HIV to uninfected gay men.

Experts were thrilled at the first compelling evidence that AIDS medications can prevent infection between men and women. The U.S. Centers for Disease Control and Prevention, which gave advice last fall for use of the preventive drugs among gays, is now developing guidance for heterosexuals in this country.

At the same time, national and international health officials said it's far from clear how preventive use of these drugs will play out: How many people would want to take a pill each day to reduce their risk of HIV infection? Would they stick with it? Would they become more sexually reckless?

Another issue: There already is a supply problem. In Africa, 6.6 million people are now on AIDS drugs, but nine million people who are eligible for the treatment are on a waiting list, according to the World Health Organization.

In the United States, many state assistance programs that help people access AIDS medications also have waiting lists.

The first of the new studies, run by the CDC, involved more than 1,200 men and women in Botswana. About half took Truvada each day. The other half got a fake pill.

An analysis of those who were believed to be regularly taking the pills found four of those on Truvada became infected with HIV, compared to 19 on the dummy pill. That means the drug lowered the risk of infection by roughly 78 percent, researchers said.

The second study was funded by the Bill & Melinda Gates Foundation and run by the University of Washington. It involved more than 4,700 heterosexual couples in Kenya and Uganda. In each couple, one partner had HIV and the other did not. The uninfected were given either daily placebos or one of the Gilead pills — Truvada or Viread.

The study found 13 HIV infections among those on Truvada, 18 in those on Viread, and 47 of those on dummy pills. So the medications reduced the risk of HIV infection by 62 percent to 73 percent, the researchers said.

“Our results provide clear evidence that this works in heterosexuals,” said Baeten, who co-chaired the study.

An independent review panel said later the benefit was clear-cut and stopped handing out placebos, instead offering the preventive drugs. Essentially, they deemed it unethical to withhold the medications from people who had been on placebo, Baeten said.

In both studies, participants also were offered counseling and free condoms, which may help explain the relatively low overall infection rate.

These are the third and fourth widely reported studies of Gilead's treatments.

The first was announced last year, involving gay men in Peru, Ecuador, Brazil, South Africa, Thailand and the United States (San Francisco and Boston). Truvada lowered the chances of infection by 44 percent — and by 73 percent or more among men who took their pills most faithfully.

Experts celebrated. The CDC advised doctors on prescribing the pill along with other prevention services for gay men, based on those encouraging results.

But momentum seemed to stall in April, when an interim analysis of a study of 3,900 women in Kenya, Tanzania and South Africa did not show a benefit from Truvada. Scientists can't explain the failure in that study but one theory is that the women did not take the pill as often as they should have, said Dr. Lynn Paxton, who has coordinated the federal agency's HIV prevention research.

Gilead Sciences is a major producer of AIDS drugs. United Nations health officials recently announced the company had agreed to allow Truvada, Viread and two other drugs to be made by generic manufacturers, potentially increasing their availability in poor countries.

AP Medical Writer Maria Cheng in London contributed to this report.