Pre-exposure prophylaxis or PrEP is the use of an antiretroviral medication to prevent the acquisition of HIV infection by uninfected persons. PrEP may either be taken orally, using an antiretroviral drug available for treatment of HIV infection (tenofovir plus emtricitabine), or topically as a vaginal gel containing tenofovir. The efficacy of oral PrEP has been shown in four randomized control trials and is high when the drug is used as directed. The efficacy of gel has been shown in one trial and is moderate. Making these drugs available for safe, effective prevention outside the clinical trial setting is the current challenge. As of September 2015, WHO recommends that people at substantial risk of HIV infection should be offered PrEP as an additional prevention choice, as part of comprehensive prevention.