PreP, or HIV pre-exposure prophylaxis, is meant of for individuals patients who are at high risk of acquiring HIV. This means patients who have receptive anal sex AND have multiple sexual partners, or relationships where one partner is HIV positive with a low or undetectable viral load and the other is negative.
It is not meant for low risk individuals who do not partake in the above activities or fit the above scenario.
A patient on PreP takes a combination anti-viral pill, Truvada, daily. Along with condom use and other safer sex practices, it can help reduce the risk of acquiring HIV. It is meant to be as one part of the overall risk reduction strategy and not a stand-alone intervention.
Anyone on PreP should be seen every 3 months at a minimum to review sexual practices, get screened for other STIs and monitor their tolerance of the medication.
Further information can be found on the CDC website at http://www.cdc.gov/hiv/basics/prep.html