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PrEP vs. PEP: What’s the Difference and What’s More Effective?

Posted on October 24, 2022
Medically reviewed by
Barry S. Zingman, M.D.
Article written by
Maureen McNulty

HIV can spread easily between people during sex or when sharing needles. However, certain medications can help stop new infections from occurring. A therapy that’s used to prevent a condition rather than treat it is known as prophylaxis.

There are two main forms of prophylaxis for HIV. Preexposure prophylaxis, or PrEP, consists of medications that reduce the chances of a future infection. People who are at risk of contracting HIV, such as those who have multiple sexual partners or use injectable drugs, can take PrEP to help protect their health. Post-exposure prophylaxis, or PEP, includes drugs taken by people who may have been exposed to the virus.

Talk to your health care team if you are interested in learning more about HIV prevention methods.

Who Takes PrEP and PEP?

Both PrEP and PEP help prevent infection, but they are used in different situations. PrEP is used before being exposed to HIV, whereas PEP is taken after an exposure.

PrEP: For Those at Risk

Doctors may recommend PrEP for people who are HIV-negative (don’t have an infection) but have a high risk of being exposed to the virus. Note that many people whose partners are HIV-positive no longer need PrEP — scientific evidence shows that people with HIV who achieve an undetectable viral load while on treatment cannot transmit the virus to others.

One of the communities affected most by the spread of HIV is men who have sex with men. According to the Centers for Disease Control and Prevention (CDC), nearly 1 out of 4 sexually active people ages 18 to 59 in this category have a significant risk of contracting HIV. Trans women who have sex with men are at even higher risk. Additionally, a little less than 1 out of 5 people who inject drugs may be at high risk. It’s especially important for people in these groups to consider using PrEP.

However, PrEP can help anyone of any gender or sexual identity. You may want to consider PrEP if you:

  • Sometimes have sex without a condom and have multiple partners, have a partner who has multiple partners, or don’t know whether your partner has HIV
  • Are sexually active with someone who is HIV-positive but are unsure if their HIV is under control (undetectable) for at least 6 months in a row or if they are continuing to take their HIV medications
  • Have had a sexually transmitted infection (STI) in the past six months
  • Inject drugs, especially if you share needles with other people
  • Sometimes have unprotected sex with people who inject drugs

It may also be a good idea to take PrEP medications if you and your partner are trying to get pregnant. If PrEP protects you from HIV during pregnancy, it will help prevent spreading the virus to the baby.

PrEP can be a good way to stay safe and healthy if your lifestyle puts you at risk of HIV. Talk to your doctor to learn more about these medications and see if they would be a good fit for you.

PEP: For Emergency Situations

You may be able to take PEP if you don’t currently have HIV but may have been exposed during an encounter. PEP may be a good idea if you have experienced the following in the past 72 hours (three days):

  • You may have come into contact with HIV during sex (for example, you had unprotected sex or the condom broke).
  • You shared needles or other drug-related equipment with someone else.
  • You were sexually assaulted.
  • You had a possible HIV exposure while working (for example, you accidentally stuck yourself with a needle while working as a health care provider).

If you think there is a chance you came into contact with HIV, take quick action. Go to an emergency room or call your doctor. The sooner you start PEP, the better.

PEP isn’t meant to be used on a regular basis. If you frequently find yourself dealing with possible exposures, PrEP may be a better option.

How To Take PrEP and PEP

There are a couple of types of PrEP:

  • An oral (taken by mouth) medication containing a mix of two drugs — Emtricitabine and tenofovir disoproxil fumarate (sold as Truvada and generic versions)
  • Another oral medication containing a slightly different combination of drugs — Emtricitabine and tenofovir alafenamide (Descovy)
  • A medication that is injected into the muscle — Cabotegravir (Apretude)

PrEP pills are usually taken once daily. However, some people take this medication “on demand,” or as needed, taking two doses two to 24 hours before sex and then one pill every 24 hours for two days after. This strategy may provide sufficient protection for people who don’t have sex often and can predict when they will have sex. Talk with your doctor before you decrease how often you take PrEP.

PrEP injections are given in a clinic or doctor’s office every two months.

You will need to undergo testing every three months while taking PrEP pills or every two months if you’re on injectable PrEP to check your HIV status and be screened for other STIs.

PEP treatment usually contains some of the same drugs found in PrEP, along with an additional drug. Doctors usually recommend taking Truvada plus raltegravir (Isentress) or another medication in the same class of HIV medicines for PEP treatment.

PEP medications are taken once a day for four weeks (28 days). You should have HIV tests while taking PEP and after completing treatment. If you have potentially been exposed to the virus, start taking PEP as soon as possible. Treatment should begin within 72 hours of exposure.

How PrEP and PEP Work

During an HIV infection, HIV enters your immune cells and produces many copies of itself. PrEP and PEP drugs help stop the copying process, helping to prevent HIV from establishing itself as a chronic infection in the body.

PrEP and PEP Effectiveness

The goal of both PrEP and PEP is to prevent new HIV infections, and each form of prophylaxis is highly effective.

PrEP can reduce your chance of an HIV infection by up to 99 percent during sex. It can also lower your infection risk by more than 74 percent during injection drug use.

PrEP is most effective when taken as prescribed, whether daily or on demand. If you use injectable PrEP, you need to keep getting the shots as scheduled. If you don’t use PrEP as recommended, your risk of an HIV infection will increase.

PrEP doesn’t prevent pregnancy or protect against STIs, so it’s still important to use latex or polyurethane condoms when having sex.

PEP treatment is also very effective. In one study, the CDC found that only 0.5 percent of people were infected with HIV after taking PEP following an HIV exposure. PEP is more likely to fail if you don’t start taking it right away or if you miss doses or continue to experience potential exposures (for example, unprotected sex) while using the medications.

Are PrEP and PEP Safe?

PrEP and PEP side effects are generally mild. In many cases, they eventually ease or disappear. Side effects of PrEP and PEP medications include:

  • Nausea
  • Headache
  • Tiredness
  • Diarrhea

Injectable PrEP may lead to symptoms at the injection site, such as tenderness and swelling, as well as:

  • Fatigue
  • Muscle aches
  • Fever

If you have any lingering side effects, tell your doctor.

PrEP may not be as safe for some people with kidney damage, and this may influence which treatment your medical provider recommends. It should also be used with caution in people who take certain other medications, such as nonsteroidal anti-inflammatory drugs (NSAIDs) like ibuprofen (Advil). Ask your doctor if PrEP can be safely combined with your current medications and which choices are best for you.

In some cases, PEP can interfere with other medications. Make sure to tell your doctor about all the medications you are taking so that they can identify any potential problems.

Learning More About PrEP

Talk to your doctor to learn more about whether taking PrEP may be a good option for you or your partner. Your health care team can help you learn more about different preventive HIV treatments.

If you’re not sure where to get PrEP or you have a hard time obtaining it from your usual doctor, try PrEPlocator.org. This website lists local providers who can prescribe the medications you need. It also displays resources to help people access PrEP when they are uninsured or need financial assistance.

Talk With Others Who Understand

On myHIVteam, the social network for people with HIV and their loved ones, more than 35,000 members come together to ask questions, give advice, and share their stories with others who understand life with HIV.

Are you interested in starting PrEP? Have you used PEP treatments? Share your experience in the comments below, or start a conversation by posting on your Activities page.

All updates must be accompanied by text or a picture.
Barry S. Zingman, M.D. specializes in HIV/AIDS medicine and general infectious disease. Review provided by VeriMed Healthcare Network. Learn more about him here.
Maureen McNulty studied molecular genetics and English at Ohio State University. Learn more about her here.

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