Overview PEP (post-exposure prophylaxis) is medicine that prevents HIV after a possible exposure. PEP must be started within 72 hours (3 days) after a recent possible exposure to HIV. Use the locator below to find PEP services near you. Remember, the sooner you start PEP, the better. Every hour counts. If you're prescribed PEP, you'll need to take it daily for 28 days. PEP is effective in preventing HIV. But to be safe, always use condoms with sex partners and use safe injection practices. PEP is not a substitute for regular use of other HIV prevention methods. And PEP is not the right choice for people who are exposed to HIV frequently.