What is it? A pill taken daily that combines tenofovir and emtricitabine, drugs that have been part of the cocktail taken by HIV patients for years.
How does it work? The drugs block an enzyme called reverse transcriptase that allows HIV cells to multiply.
Who should take it? People who are HIV-negative but risk infection. Mostly men who have sex with men, but also intravenous drug users who share equipment.
How effective is it? Up to 92%, although doctors say patients should still use condoms.