HIV drugs can be used to boost protection against HIV as well as treating symptoms after infection, research suggests.
Two studies in Africa add weight to previous data showing drugs used to treat HIV can reduce infection risk when taken daily.
The World Health Organization said the studies could have "enormous impact" in preventing HIV transmission.
The findings were revealed in the run-up to an AIDS conference in Rome.
"This is a major scientific breakthrough which re-confirms the essential role that antiretroviral medicine has to play in the AIDS response," said Michel Sidibé, executive director of the Joint United Nations Programme on HIV/AIDS (UNAIDS).
"These studies could help us to reach the tipping point in the HIV epidemic."
One trial, carried out by the University of Washington, US, followed almost 5,000 couples in Kenya and Uganda, where one person had HIV infection and the other did not.
The uninfected person took a daily HIV medicine (tenofovir), a combination of two HIV drugs (tenofovir and emtricitabine), or a placebo pill.
There were 62% fewer HIV infections in the group on the single drug and 73% fewer HIV infections in the group that took the combination, compared with those given a dummy pill.
The other trial, conducted by the United States Centers for Disease Control, followed 1,200 HIV-negative heterosexual men and women in Botswana. They received either a once-daily combination tablet or a placebo. The HIV medication reduced the risk of getting HIV by about 63% overall.
A previous trial found the combination of two HIV drugs reduced the risk of infection in gay and bisexual men by 44%. But a similar study in women at risk of HIV infection in Kenya, Tanzania and South Africa produced disappointing results.
The HIV charity, the Terrence Higgins Trust, described the latest findings as "genuinely exciting".
Head of Policy Lisa Power told the BBC: "Pre-exposure prophylaxis is not going to be available overnight but we are exploring whether it is one of a range of things that can drive down onward transmission of HIV."
But she stressed that the findings needed to be properly tested and trialled.
"If you're currently trying to stay HIV negative don't give up on the condoms yet."
The WHO and UNAIDS recommend that people make evidence-informed decisions on HIV prevention options.
They say no single method is fully protective against HIV and antiretroviral drugs for prevention need to be combined with other HIV prevention methods such as condoms.
"Effective new HIV prevention tools are urgently needed, and these studies could have enormous impact in preventing heterosexual transmission," said Dr Margaret Chan, the WHO's Director-General.
"WHO will be working with countries to use the new findings to protect more men and women from HIV infection."
Full details of the studies will be presented at the meeting in Rome.