There’s no question that a simple regimen of a single daily pill can slash HIV infections in people at risk. But although millions of people around the world could benefit, only 200,000 are prescribed what’s called pre-exposure prophylaxis, or PrEP, and 75% of them are in the United States. The disparity was the focus of anguished discussions this week at the largest annual U.S. HIV/AIDS conference. “I’m frustrated,” says Linda-Gail Bekker, a researcher at the University of Cape Town in South Africa. “We have nearly 1700 infections every week in young women and girls, and this might be a way to throttle this.”

PrEP, which uses antiretroviral (ARV) pills made for treatment as preventives, faces multiple obstacles, as Bekker and other discussed at the Conference on Retroviruses and Opportunistic Infections here. Many countries already struggle to provide ARVs to people who are infected and say they can’t afford PrEP for the large uninfected populations who need it most. People at substantial risk of becoming infected by HIV may not know about PrEP or may reject it, as several presentations highlighted. And PrEP raises touchy issues including teen sexuality and the appearance of advocating risky behavior.

Yet PrEP works. Its effectiveness in several clinical trials led the U.S. Food and Drug Administration to approve preventive use of ARVs in 2012. The World Health Organization in 2015 recommended PrEP for everyone at “substantial risk” of becoming infected—a total of tens of millions of people, to judge from the nearly 2 million new HIV infections in 2016. In some countries, including the United States, the targeted groups are “key populations” such as men who have sex with men (MSM) and sex workers. In sub-Saharan Africa, where most people become infected from heterosexual sex and many have no “high-risk” behavior, large swaths of the population are candidates for PrEP. Read more via Science Mag