Preventing HIV transmission by giving antiretrovirals to people at high risk of contracting the virus, known as pre-exposure prophylaxis, or PrEP, may be cost-effective in some settings. Researchers analyzed 13 different studies that evaluated PrEP's impact in various populations, including heterosexual couples, men who have sex with men and injection drug users. The studies were conducted in regions such as southern Africa and countries that included Ukraine, the United States and Peru. The investigators found that giving PrEP to higher risk groups appeared to be the most cost-effective prevention strategy. They also found that behavior change—such as reducing the number of sexual partners—as well as adherence to PrEP were key determinants of its effectiveness.