PrEP and PEP: HIV Prevention Explained
PrEP and PEP are among the most effective tools we have for preventing HIV. They are easy to confuse because both involve taking antiretroviral medicine, but the difference comes down to timing: PrEP is taken before exposure, PEP is taken after.
PEP: emergency prevention after an exposure
Post-exposure prophylaxis (PEP) is a short course of HIV medicine, usually taken for 28 days, that can stop an infection from taking hold after a possible exposure.
PEP is appropriate after events such as a condom breaking with a partner of unknown or positive status, sexual assault, or a needle-stick. If you think you need it, go to a doctor, urgent care, or emergency department right away and ask for PEP by name. If you find yourself needing PEP repeatedly, that is a strong sign to consider PrEP.
PrEP: ongoing protection before exposure
Pre-exposure prophylaxis (PrEP) is medicine taken by people who do not have HIV to prevent getting it. Taken as prescribed, PrEP reduces the risk of getting HIV from sex by about 99%, and from injection drug use by at least 74%. It comes as a daily pill and, for some people, as a long-acting injection given every two months.
Who might consider PrEP?
- People with an HIV-positive partner, especially if that partner is not yet virally suppressed
- People who do not consistently use condoms with partners of unknown status
- People who have had a recent sexually transmitted infection
- People who share injection equipment
PrEP requires an HIV test before starting and regular follow-up testing, because it is only for people who are HIV-negative.
U=U: treatment as prevention
There is a third piece worth knowing. A person living with HIV who takes treatment and maintains an undetectable viral load cannot transmit HIV to sexual partners. This is called Undetectable = Untransmittable, or U=U. It means effective treatment is also effective prevention.
How these fit with your risk estimate
Prevention changes the math. If a partner is on effective treatment and undetectable, or if you are on PrEP, the real-world risk of a given exposure is far lower than the baseline per-act figures in our risk guide. If an exposure has already happened and it was recent, focus first on whether PEP applies, then on testing.
Getting PrEP or PEP
Both are prescription medicines. Talk to a clinician, a sexual health clinic, or use the resources at CDC PrEP and CDC PEP. Cost-assistance programs exist in many regions.