A new study shows that taking just a few anti-AIDS pills before and after sex can reduce the risk of HIV infection by 86%, USA Today reports.

The study was published online Tuesday in The New England Journal of Medicine. A preliminary version of the study's results was presented in February at the annual meeting of the Conference on Retroviruses and Opportunistic Infections.

The new study adds to a growing list of HIV prevention strategies.

For the research study, the participants took two pills a few hours before a sexual encounter, a third the next day and a fourth the day after that.

Participants in the 400-person study took an average of 15 pills a month. The researchers followed the participants for an average of 9.3 months.

Earlier studies have proved that people can reduce their risk of HIV infection by taking a daily pill called Truvada. This prevention strategy, called pre-exposure prophylaxis, or PrEP, has been shown to reduce the risk of HIV transmission by 92% in studies of men who have sex with men.

Also, PrEP reduced the risk of HIV infection by 70% in a study of intravenous drug users.

However, doctors did not know until now that taking PrEP "on demand" was as effective as taking the pills everyday.

In more recent years, medications have become a critical part of HIV prevention.

The new study included only men who have sex with men or transgender women, who were born male but identify as female. Men in the study also had frequent sex.