LONDON (Reuters) - AIDS drugs used to treat HIV can also be used to prevent infection among heterosexual men and women, according to the results of two studies conducted in Africa released on Wednesday.

The findings add to growing evidence that drugs used since the mid-1990s to treat people who are already sick may also hold the key to slowing the spread of the sexually transmitted disease.

The research involving couples in Kenya, Uganda and Botswana found that giving daily AIDS drugs reduced infection rates by at least 62 percent when compared with placebo.

The larger of the two studies examined 4,758 couples in Kenya and Uganda in which one partner was HIV-positive and one was negative. Those negative partners taking Gilead Sciences Inc's tenofovir, or Viread, had on average 62 percent fewer infections.

For couples on Truvada -- another Gilead drug combining tenofovir and emtricitabine -- the infection risk was cut by an estimated 73 percent in the study, which was led by researchers at the University of Washington.

The study was funded by the Bill & Melinda Gates Foundation, whose director of HIV and Tuberculosis, Stefano Bertozzi, said it marked "a significant milestone in the quest to develop new HIV prevention measures."

The second study, involving just over 1,200 sexually active men and women in Botswana, found those on daily Truvada reduced their risk of HIV infection by 62.6 percent.

The idea of such "pre-exposure prophylaxis," known as PrEP, has gained traction in the past year, following results of other research showing a fall in infection rates among gay men taking AIDS drugs.

(Reporting by Ben Hirschler; Editing by Will Waterman)

Mochila insert follows.