Want to receive publications straight to your inbox?


What is PEP?

Post-exposure prophylaxis, or PEP, can be used by people who are HIV negative to help prevent getting HIV after a potential exposure to HIV. PEP is a combination of three HIV medications that an HIV-negative person takes orally for 28 days to lower their chance of getting HIV.

PEP needs to be started as soon as possible (up to a maximum of 72 hours) after a potential exposure to HIV. The sooner PEP is started, the more likely it is to work. PEP is meant to be used to prevent HIV transmission from a single accidental exposure to HIV. PEP should not be used regularly as an HIV prevention strategy. If a person uses PEP more than once, they may be a good candidate for PrEP (pre-exposure prophylaxis).

What is the difference between PEP and PrEP?

PEP is not the same as PrEP, which involves taking two HIV medications on an ongoing basis.

Differences between PEP and PrEP



Taken as soon as possible after a potential exposure to HIV (up to a maximum of 72 hours)

Taken before and after potential exposure to HIV

Taken every day for 28 days

Usually taken every day on an ongoing basis

A combination of three HIV medications

A combination of two HIV medications

Intended to be used to prevent HIV transmission from a single accidental exposure

Intended for regular use as an ongoing HIV prevention method

If you have had a potential exposure to HIV within the last 72 hours, then PEP might be right for you. The sooner you start PEP, the more effective it is.

PEP can be used after a potential exposure to HIV through sexual or injection drug use activities. This can include having unprotected sex (whether consensual or non-consensual), having a condom break during sex or sharing equipment used to inject drugs.

PEP can also be used after a potential exposure to HIV at work, such as when a healthcare worker or emergency responder has an accidental needle-stick injury.

A doctor or nurse practitioner will help you determine if you should start PEP, based on the nature of your exposure. It’s important to be honest about your potential exposure so the healthcare provider can properly assess your risk.  PEP is usually only recommended if the potential exposure carries a high or moderate chance of passing HIV.

PEP can be used by people of all genders. Experts believe that PEP can effectively prevent HIV in trans people and that the drugs in PEP are unlikely to interfere with the hormones that some trans people take; however, this has not been well studied. 

PEP can be taken by pregnant people safely. Tell your doctor if you are pregnant or planning to have a baby so they can prescribe a drug combination that is safe for you. Breastfeeding (also called chestfeeding) is not recommended while taking PEP.

Is PEP safe?

Some people who take PEP experience temporary side effects such as nausea, fatigue and diarrhea. For most people, side effects are mild and manageable. Side effects can vary depending on the type of drugs prescribed for PEP and the person who is taking them. Talk to your doctor or nurse practitioner if you are having side effects from PEP.

How can I get PEP?

PEP needs to be prescribed by a healthcare provider. If you think you have been exposed to HIV, you should immediately contact your doctor or nurse practitioner, a hospital emergency room or a sexual health clinic to ask about where to access PEP.

The healthcare provider will ask you questions to determine if you need PEP. Sometimes a “starter pack” (a partial supply) of PEP is provided to help you start PEP right away.

The HIV medications used in PEP are often expensive, and coverage for the cost varies across Canada. If your potential exposure happened while you were working, PEP is usually covered. PEP is covered by some public and private health insurance plans after a potential exposure through sexual and drug use activities. However, this varies by province or territory and can also vary depending on the type of exposure (e.g., sexual assault versus consensual sex). PEP is often covered if it is necessary after a sexual assault.

You may want to ask your doctor or nurse practitioner, a pharmacist, a sexual health clinic or an HIV organization about ways to help cover the cost of PEP.

What will happen before and after I take PEP?

Before starting PEP you will be tested for HIV. It is important to know your HIV status to make sure you are given the correct medication — PEP is only for HIV-negative people, and if you have HIV you need HIV treatment. If you test positive for HIV after starting PEP, you should discuss with a healthcare provider how you can safely stop PEP and begin HIV treatment.

Before starting PEP you will have your kidney (and possibly liver) function tested, and you should be tested for hepatitis A, B and C. Your healthcare provider may recommend testing for other sexually transmitted infections (STIs), depending on the nature of your exposure. You may need ongoing monitoring if any results are abnormal or if you experience side effects from PEP medications.

People on PEP need to take PEP medications every day for 28 days. Talk to your doctor, nurse practitioner or pharmacist if you think you will have trouble remembering to take your pills every day. They may be able to help you come up with a strategy that will work for you.

After taking PEP, you will need to be tested for HIV to assess whether PEP has worked. Retesting usually happens six to 12 weeks after the potential exposure to HIV.

If you have an ongoing chance of getting HIV, PrEP could be a good prevention option for you. Talk to your doctor or nurse practitioner about whether PrEP is right for you.

How well does PEP work to prevent HIV transmission?

PEP does not prevent 100% of HIV infections but it is very effective at preventing HIV if used every day for 28 days. PEP should ideally be started right away, but it can be started up to a maximum of 72 hours after a potential exposure to HIV. The sooner you start PEP, the more effective it is.  

To maximize the effectiveness of PEP:

  1. Get PEP prescribed by a healthcare provider
  2. Start taking PEP as soon as possible after an exposure (up to 72 hours after)
  3. Take PEP medications as prescribed, typically every day for 28 days
  4. Use other prevention strategies (e.g., condoms or new needles) to avoid other potential exposures while taking PEP

What are some other ways to help prevent HIV and other infections?

PEP is the only way to help prevent HIV after a potential exposure to HIV. It is for emergency use only.  It is not meant to be used as a long-term HIV prevention strategy.

There are several ways to help prevent HIV before you are exposed to HIV:


PrEP is a highly effective way to prevent getting HIV when used as prescribed, on an ongoing basis, starting before and continuing after exposure to HIV.


Condoms are highly effective at preventing both HIV and other sexually transmitted infections (STIs) if used consistently and correctly.

New drug use equipment

For people who inject drugs, using new injection equipment each time is a highly effective strategy to prevent HIV, hepatitis B and C, and other infections.

Maintaining an undetectable viral load

When people with HIV take treatment and maintain an undetectable viral load, they do not pass HIV on to their sex partners. Their risk of passing HIV through sharing needles is also reduced, but we don’t know exactly how much. If you have a partner with HIV who is on treatment and maintaining an undetectable viral load, this is a highly effective strategy to prevent the sexual transmission of HIV.

Universal precautions

In a work context, it is important to use universal precautions, such as wearing gloves, disinfecting surfaces and handling and discarding sharps safely, to avoid coming into contact with bodily fluids that may transmit HIV or other infections.

CATIE thanks the Halifax Sexual Health Centre, Women and HIV/AIDS Initiative (WHAI), and Action Canada for Sexual Health and Rights for reviewing this resource. We also thank Patrick O’Byrne, University of Ottawa, for medical review.