PrEP or PEP: which one is the question you are actually asking

PrEP and PEP are both HIV medication used by people who do not have HIV, to stop them getting it. They are named almost identically, which is unhelpful, because choosing between them is not a preference. It is decided entirely by where you are standing relative to an exposure.

PEP is post-exposure prophylaxis. Something has already happened, and you are trying to stop an infection taking hold before it does. It is a twenty-eight day course, and it has a hard deadline: it must begin within seventy-two hours of the exposure, and the sooner inside that window it starts, the better it works. Past seventy-two hours it is not offered, because it no longer does the thing it exists to do. If you are reading this because of something that happened last night, stop reading and call a clinic.

PrEP is pre-exposure prophylaxis. Nothing in particular has happened, and you are making sure that when something does, it does not turn into an infection. Taken as prescribed, it reduces the risk of getting HIV from sex by about ninety-nine per cent, which makes it one of the most effective preventive medications in routine use. It is taken ongoing rather than as a course, and it needs a negative HIV test before it starts and periodic check-ups — usually about every three months — while it continues.

The distinction matters because the two are not interchangeable in either direction. PrEP does not work retroactively: taking it after an exposure is not a substitute for PEP, and treating it as one wastes the seventy-two hours that were the whole opportunity. In the other direction, PEP is not a strategy. If you find yourself needing it more than occasionally, that is not a sequence of emergencies — that is a conversation about PrEP that has not happened yet, and most people who start PrEP arrive at it exactly this way.

Neither protects against other sexually transmitted infections. That is not a footnote — it is why PrEP care includes regular screening rather than being a prescription you collect and forget. The medication removes one risk very effectively and leaves the others exactly where they were.

If you are not sure which of these applies to you, the deciding question is simply whether something has already happened and when. If it was in the last seventy-two hours, it is PEP and it is urgent. Otherwise, there is time to sit down and talk it through properly.