HIV Prevention

What is post-exposure prophylaxis?

Post-exposure prophylaxis (PEP) refers to the use of HIV medicines to prevent HIV infection shortly after a possible exposure. PEP must be started within 72 hours (three days) after possible exposure to work effectively.

Although PEP can help prevent HIV transmission, it should be used in emergencies only and is not meant for regular use by people who may be exposed to HIV frequently. PEP is not intended to replace the regular use of other HIV prevention methods, such as consistent and proper use of condoms during sex or pre-exposure prophylaxis (PrEP).

PrEP is different from PEP because it is taken regularly before someone is exposed to HIV. PrEP can be taken as a daily pill, or as an injection less frequently. For more information, see the HIVinfo fact sheet on Pre-Exposure Prophylaxis (PrEP).

Who should consider taking PEP?

PEP can be given to people who do not have HIV or who do not know their HIV status and who may have been exposed in the last 72 hours. This includes people who—

• May have been exposed to HIV during sex
• Shared needles or other equipment to inject drugs
• Were sexually assaulted
• May have been exposed to HIV at work (an occupational transmission)
• Had any other suspected exposure (such as a needlestick injury)

If you think you were recently exposed to HIV, talk to your health care provider or an emergency room doctor about PEP immediately. The sooner PEP is started after possible HIV exposure, the better. Every hour counts.

What HIV medicines are used for PEP?

CDC provides guidelines on recommended HIV medicines for PEP, which include recommendations for specific groups of people, such as pregnant women or people with kidney problems.

The HIV medicines used for PEP are also commonly used for HIV treatment. When PEP is started soon after exposure, it prevents the virus from replicating or creating a latent HIV reservoir. Learn more in the HIVinfo fact sheets, The Stages of HIV Infection and What is a Latent HIV Reservoir?.

A health care provider will work with you to determine which PEP medicines are the most likely to be safe and effective for you.

How well does PEP work?

CDC’s preferred PEP regimens are most effective when started right away and taken every day for 28 days. In five studies using bictegravir- or dolutegravir-based PEP, no participants acquired HIV.

Although PEP can be very effective, its success depends on taking all medicines exactly as prescribed, avoiding new exposures while on PEP, and using other prevention methods during and after finishing the 28-day course. If you may be exposed to HIV again after completing PEP—such as if you have a partner with HIV—your health care provider may recommend starting PrEP to support long-term prevention.

While taking PEP, it is important to keep using other HIV prevention methods, such as using condoms correctly every time you have sex, and using new, sterile drug injection equipment only. PEP does not protect against other sexually transmitted infections (STIs), so condoms can help reduce the risk of HIV and other STIs. 

Does PEP cause side effects?

The HIV medicines used for PEP are well-tolerated but may cause side effects in some people. The most common side effects are generally mild and may include diarrhea, nausea, tiredness, and headache. Mild side effects can usually be treated or go away on their own.

Less commonly, PEP medicines can cause more serious side effects, such as liver problems or lactic acidosis that may require medical attention.

Before taking PEP, be sure to consult your health care provider about any potential side effects caused by PEP medicines. Contact your health care provider immediately if you have side effects that make it hard for you to keep taking your PEP medicines.

What should you do after taking PEP?

After completing a full course of PEP, you should get tested to confirm whether the medicines successfully prevented possible HIV transmission.

If you test negative for HIV after finishing PEP, you should continue using HIV prevention methods, such as taking PrEP (if needed) and using condoms. See The Basics of HIV Prevention for more tips to prevent HIV.

This fact sheet is based on information from the following sources:

From CDC:

• Antiretroviral Postexposure Prophylaxis After Sexual, Injection Drug Use, or Other Nonoccupational Exposure to HIV — CDC Recommendations, United States, 2025
• Preventing HIV with PEP
• Now’s the Time to Find Out About PrEP and PEP (PDF)

From MedlinePlus:

• HIV: PrEP and PEP

Also see the HIV Source collection of HIV links and resources.

What is pre-exposure prophylaxis?

Pre-exposure prophylaxis (PrEP) refers to HIV medicine taken by people without HIV to reduce the risk of HIV transmission before a possible exposure. The word “prophylaxis” means to prevent or control the spread of an infection or disease.

Four HIV medicines are approved by the U.S. Food and Drug Administration (FDA) for use as PrEP: 

• Descovy (or a generic equivalent), a pill taken by mouth every day.
• Truvada (or a generic equivalent), a pill taken by mouth every day.
• Apretude, a shot taken every two months.
• Yeztugo, a shot taken every six months.

Which medicine to use for PrEP depends on a person’s individual situation.

PrEP works by blocking the HIV life cycle. This prevents the virus from taking hold and spreading if exposed during sex or injection drug use. However, its effectiveness relies on taking the medication as prescribed to ensure there is enough medicine in the bloodstream to stop the virus from spreading.

Should I consider taking PrEP?

PrEP is used by people who do not have HIV and who may be exposed to HIV through sex or injection drug use. PrEP is not intended for people with HIV or who have recently been exposed to HIV.

According to the Centers for Disease Control and Prevention (CDC), PrEP may be beneficial if you do not have HIV and—

• You have had anal or vaginal sex in the past six months and—
  • Have a sexual partner with HIV (especially if the partner has an unknown or detectable viral load),
  • Have not consistently used a condom, or
  • Had a sexually transmitted infection (STI) in the past six months
• You inject drugs and—
  • Have an injection partner with HIV, or
  • Share needles, syringes, or other injection equipment
• Have been prescribed post-exposure prophylaxis (PEP) and—
  • Report continued sex, or
  • Have used multiple courses of PEP

If you think PrEP may be right for you, talk to your health care provider. PrEP can only be prescribed by a provider, and you’ll need to take other tests to make sure it’s safe and appropriate for you.

How well does PrEP work?

According to the CDC, PrEP pills and shots can reduce the risk of sexual HIV transmission by about 99% when taken exactly as prescribed. PrEP pills can reduce the risk of HIV transmission from drug injections by as much as 74%. PrEP shots have not been studied for this type of HIV exposure. It is important to avoid sharing needles, syringes, or other injection equipment even while taking PrEP.

Although getting HIV is unlikely when taking PrEP as prescribed, it can still occur. In rare cases, people who get HIV while taking PrEP may develop HIV drug resistance. People who develop drug resistance may have fewer treatment options. However, a health care provider can still help identify effective options. Taking PrEP exactly as prescribed remains one of the safest and most effective ways to prevent HIV.

PrEP does not protect against other STIs. Combining other prevention methods (such as condom use) with PrEP can further reduce the risk of HIV or other STIs.

Does PrEP cause side effects?

Some people taking PrEP may experience side effects such as nausea, diarrhea, headache, dizziness, and insomnia. These side effects are typically mild and go away over time. In rare cases, more serious side effects such as kidney failure and lactic acidosis can occur.

Before starting PrEP, consult with your health care provider about any potential side effects. If you are taking PrEP, tell your health care provider immediately if you have any side effects that make it difficult to keep taking your PrEP medicines.

How often should you take PrEP?

PrEP pills: PrEP pills (Truvada or Descovy) should be taken once a day. This is the only CDC- and Food and Drug Administration (FDA)-recommended way to take PrEP pills for HIV prevention.

Some people may take PrEP “on demand” only when they have a high chance of HIV exposure. On-demand PrEP refers to a “2-1-1” schedule, in which you take two PrEP pills between two and 24 hours before sex, one pill 24 hours after the first dose, and another pill 24 hours after the second dose. 

Because on-demand PrEP is not recommended by CDC or approved by FDA, this approach should only be considered if recommended by your health care provider. 

PrEP shots: PrEP shots should be received every two months (Apretude) or every six months (Yeztugo). Depending on the type of shot, you may also be required to have more frequent shots or take pills for a couple of days when you first start injectable PrEP.

Whether taking PrEP pills or shots, it is important to take all doses on time. PrEP is much less effective when it is not taken as prescribed, which increases the likelihood of HIV transmission and drug resistance.

What else should you know about PrEP?

People taking PrEP should be tested for HIV regularly (usually every three months), which may require routine follow-up visits with a health care provider. If taking PrEP as scheduled doesn’t work for you or if you want to stop taking PrEP altogether, talk to your health care provider first before making changes.

You are unlikely to get HIV if you take PrEP as prescribed. If you do get HIV while taking PrEP, you will need different medicines for treatment. For information on what to do if you test positive for HIV, see the HIVinfo fact sheet Just Diagnosed: Next Steps After Testing Positive for HIV.

This fact sheet is based on information from the following sources:

From CDC:

• Clinical Guidance for PrEP
• Let’s Stop HIV Together: PrEP
• Preventing HIV with PrEP
• PrEP Medication Guide: Reduce Your Risk of Getting HIV
• Now’s the Time to Find Out About PrEP and PEP

From DailyMed:

• Truvada
• Descovy
• Apretude
• Yeztugo

Also see the HIV Source collection of HIV links and resources.

How is HIV transmitted?

The person-to-person spread of human immunodeficiency virus (HIV) is called HIV transmission. People can get or transmit HIV only through specific activities, such as sex or injection drug use.

In the United States, HIV is transmitted mainly by:

• Having anal or vaginal sex with someone who has HIV without using a condom or who is not taking medicines to prevent or treat HIV.
• Sharing injection drug equipment (“works”), such as needles or syringes, with someone who has HIV.

View the HIVinfo Understanding How HIV is Transmitted fact sheet and the You Can Safely Share…With Someone With HIV infographic for more information on how HIV transmission can and cannot occur.

How can I reduce the chances of getting HIV?

Anyone can get HIV, but certain actions can help reduce the likelihood of HIV transmission.

• Get tested. Talk to your partners about testing for HIV and other sexually transmitted infections (STIs) before you have sex and on an ongoing basis. Having an untreated STI can increase the likelihood of HIV transmission. Use the GetTested locator to find a testing location near you.
• Choose safer sexual behaviors. HIV is mainly transmitted by having anal or vaginal sex without a condom or without taking medicines to prevent or treat HIV.
• Use condoms correctly every time you have sex. Condoms help prevent the spread of STIs, including HIV. Read this fact sheet from CDC on condom use.
• Know your sexual partners. The likelihood of coming into contact with HIV or another STI may increase when you have more sexual partners or partners you aren’t familiar with.
• Talk to your health care provider about pre-exposure prophylaxis (PrEP). PrEP is an HIV prevention option if you do not have HIV but are at an increased risk of getting HIV (for example, if your partner has HIV or if you inject drugs). For more information, read the HIVinfo fact sheet on Pre-Exposure Prophylaxis (PrEP).
• Do not share injection drug equipment. Because HIV is transmitted through blood, injecting drugs using shared equipment can significantly increase the odds of HIV transmission. Never share drug injection equipment with others.

If I have HIV, how can I prevent passing it to others?

Take HIV medicines as directed by your doctor. Treatment with HIV medicines (called antiretroviral therapy or ART) helps people with HIV live long, healthy lives. ART cannot cure HIV, but it can reduce the amount of HIV in the body (called the viral load).

One of the main goals of ART is to reduce a person's viral load to an undetectable level. An undetectable viral load means that the level of HIV in the blood is too low to be detected by a viral load test.

People with an undetectable viral load have no risk of transmitting HIV to sexual partners, a concept known as Undetectable = Untransmittable (or U=U). People with a detectable viral load can transmit HIV, even if they are taking ART. For this reason, regular viral load testing is recommended.

Although HIV medicines can reduce the chances of HIV transmission, they do not prevent transmission of other STIs. 

In addition to maintaining an undetectable viral load, other steps you can take to make sure you prevent HIV transmission to others include:

• Using condoms correctly every time you have sex
• Communicating about your HIV status
• Talking to your partners about taking PrEP
• Attending all follow-up appointments
• Not sharing drug injection equipment

How else are HIV medicines used for prevention?

In addition to ART and HIV PrEP, HIV medicines are also used for post-exposure prophylaxis (PEP) and to prevent perinatal transmission of HIV.

• Post-exposure prophylaxis (PEP) 
  PEP refers to taking a short course (28 days) of HIV medicines to prevent HIV transmission, starting within 72 hours after a possible exposure. PEP should not be used regularly. For more information, read the HIVinfo fact sheet on Post-Exposure Prophylaxis (PEP).
• Prevention of perinatal transmission of HIV 
  HIV medicines are used during pregnancy, childbirth, and while breastfeeding to maintain an undetectable viral load, which significantly lowers any risk of perinatal transmission of HIV. After birth, infants receive HIV medicine to reduce the chances of transmission if exposure occurred during childbirth. For more information, read the HIVinfo fact sheet on Preventing Perinatal Transmission of HIV During Pregnancy and Childbirth.

This fact sheet is based on information from the following sources:

From CDC:

• Preventing HIV
• Preventing HIV with PrEP
• Preventing HIV with PEP

From the HIV Clinical Practice Guidelines at Clinicalinfo.HIV.gov:

• Recommendations for the Use of Antiretroviral Drugs During Pregnancy and Interventions to Reduce Perinatal HIV Transmission in the United States:
  • Antepartum Care for Individuals With HIV: Overview
  • Care of Infants With Perinatal Exposure to HIV: Antiretroviral Management of Infants With In Utero, Intrapartum, or Breastfeeding Exposure to HIV

Also see the HIV Source collection of HIV links and resources.