Stony Brook Medicine Health News
PrEP and PEP for HIV

PrEP and PEP for HIV Prevention

PrEP and PEP are valuable tools in preventing HIV transmission. Both of these medications are used to reduce the risk of HIV infection, although they’re best suited for different situations. If you’re concerned about exposure to HIV, contact your healthcare provider or a local STI clinic to discuss your best course of action.  

PrEP and PEP: What Are They?

Pre-exposure prophylaxis (PrEP) and post-exposure prophylaxis (PEP) are two important strategies for preventing HIV transmission.

PrEP involves taking a daily medication to reduce the risk of HIV infection in people who are at high risk of exposure to the virus. It is highly effective when taken as prescribed and can significantly lower the chances of acquiring HIV.

PEP, on the other hand, involves taking a combination of antiretroviral medications within 72 hours of potential exposure to HIV to prevent infection. PEP is intended for emergency situations, such as condom breakage or unprotected sex with a partner who is HIV positive or has an unknown HIV status.

How Do PrEP and PEP Work?


PrEP works by using antiretroviral medications to block HIV from establishing itself and multiplying in the body. The medication used for PrEP is a combination of two drugs: tenofovir and emtricitabine.

When taken consistently, PrEP can reduce the risk of HIV infection by more than 90% in people who are at high risk of exposure to the virus. For PrEP to be most effective, it needs to be taken every day as prescribed.


While PrEP is taken daily, PEP is taken for 28 days immediately after HIV exposure. It is intended for emergency situations and is not used long-term. 

PEP involves taking a combination of antiretroviral medications to prevent HIV infection from taking hold in the body. It is essential to follow the prescribed regimen exactly as directed for PEP to be most effective.

Who Should Consider Taking PrEP and PEP?

PrEP is recommended for those who are at high risk of HIV infection, including:

  • People who are in a sexual relationship with an HIV-positive partner
  • Those who engage in condomless sex with multiple partners, particularly if their partners’ HIV status is unknown
  • Injection drug users who share needles or other injection equipment
  • Sexually active individuals who have been diagnosed with a sexually transmitted infection (STI) within the past six months

You can consider taking PEP if you have been:

  • Exposed to HIV through unprotected sex with a partner of unknown HIV status or a partner known to be HIV-positive
  • Exposed to HIV through sharing needles or other injection equipment with someone who has HIV
  • The victim of sexual assault or rape

If you suspect you’ve been exposed to HIV and think PrEP or PEP might be right for you, speak with a healthcare provider right away, who can offer a consultation and the right prescription based on your situation. 

Visit an STI Clinic or healthcare provider as soon as possible after suspected HIV exposure, as PEP should be started ideally within hours of exposure and no later than 72 hours after. 

At the Stony Brook STI Center, we offer comprehensive STI testing, and Prep and PEP therapies. If you suspect that you have already contracted a sexually transmitted illness, our STI Center offers quick testing, diagnosis and treatment. 

This article is intended to be general and/or educational in nature. Always consult your healthcare professional for help, diagnosis, guidance and treatment.