PEP is a month long course of HIV drugs that someone takes very soon after sex which had a risk of HIV transmission.
The drugs are the same ones taken by people with HIV, and for PEP to work they must be taken for four weeks. The sooner PEP is started, the more likely it is to work; within 24 hours is best, but no later than 72 hours (three days). After 72 hours PEP is unlikely to work.
PEP is not a ‘morning after’ pill to stop HIV as it is not taken just once but must be taken every day for 28 days. If someone stops taking it before 28 days there is a possibility that it will not have worked.