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Post-exposure prophylaxis (PEP)

Dennis Boyd | February 16, 2022

PrEP vs. PEP

On-demand PrEP

What is PEP?

PEP consists of a combination of three HIV medications that an HIV-negative person takes for four weeks to reduce their risk of getting HIV after a potential exposure to HIV. This is different from pre-exposure prophylaxis (PrEP), which involves taking two HIV medications on an ongoing basis, starting before and continuing after an exposure to HIV.

PEP should be started as soon as possible, but definitely within 72 hours of being exposed to HIV. PEP is commonly accessed through an emergency room visit. If you think you need PEP go to a hospital or emergency room straight away. The prescription drugs used for PEP need to be taken every day for four full weeks (28 days).

How does PEP work to help prevent HIV?

How does PEP work to help prevent HIV?

PEP interferes with the pathways that HIV uses to cause a permanent infection in the body. For HIV to cause infection the virus must enter the body, infect certain immune cells, make copies of itself (replicate) within these immune cells, then spread throughout the body.

When PEP is taken, the HIV drugs get into the bloodstream and the genital and rectal tissues. If there is HIV in the body, the drugs can prevent HIV from replicating within the body’s immune cells, and help to prevent a permanent infection from developing.

PEP drugs need to start working against HIV as soon as possible after the virus enters the body, meaning that PEP should be started as soon as possible after a potential exposure and not more than 72 hours afterwards. Drug levels must also remain high during the month of treatment to help prevent infection. If the pills are not taken consistently, as prescribed, there may not be enough medication in the body to prevent HIV infection.

When is PEP used?

When is PEP used?

PEP can be used after exposure to HIV in a work context (occupational PEP) or after exposure to HIV that is not work-related such as sexual exposure or injection drug use (non-occupational PEP or nPEP).

Occupational PEP is when PEP is used by people who have exposure to blood and/or body fluids that may contain HIV in their workplace — for example, a healthcare worker who accidentally experiences a needle-stick injury.

Non-occupational PEP is when PEP is used after a potential high-risk exposure to HIV that is not work-related, such as unprotected sex, a condom breaking during sex, sexual assault, or sharing needles used to inject drugs.

What is involved in taking PEP?

What is involved in taking PEP?

First, a doctor or nurse will assess whether the risk of HIV transmission is high or low. If the risk is high enough, PEP will be prescribed.

PEP ought to be used by people who are HIV negative. When a person starts PEP, an HIV test is done to determine their HIV status. If the person is HIV positive (but doesn’t know it) they should be referred to HIV care and treatment.

If rapid HIV testing (which gives results within a matter of minutes) is not available, the test result may not be ready for one to two weeks; however, PEP will be started right away. PEP should be discontinued if the PEP user tests HIV positive, or if the contact person is confirmed to be HIV negative.

PEP medications ought to be taken consistently and correctly—every day for four weeks—or the risk of HIV infection will increase. A counsellor, doctor, nurse, pharmacist, or staff member at an AIDS Service Organization may suggest strategies to help a person adhere to the pill-taking schedule and/or manage any side effects of the drugs.

A person taking PEP would often need monitoring for side effects and other complications such as drug toxicity, though this is rare. Blood tests may be needed to ensure that the medications are not causing harm to the body. If side effects and toxicity are a problem, a doctor may decide to change one or more of the drugs being used for PEP.

A person taking PEP should take extra precautions to avoid exposure to HIV while taking PEP. The use of PEP is intended to reduce the risk of infection associated with one exposure.

How well does PEP work?

How well does PEP work?

Observational research suggests that PEP can reduce the risk of getting HIV by more than 80%, which means some people in the studies acquired HIV despite taking PEP. Although some of these people reported high adherence to PEP and no further exposures to HIV, many HIV transmissions among people taking PEP occurred due to low adherence (not taking PEP every day for 28 days) and/or ongoing exposures to HIV. Effectiveness is likely much higher than 80% if PEP is used consistently and as prescribed.

This would include:

  • Obtaining PEP from a healthcare provider.
  • Taking the medications every day for 28 days. High adherence to the full month-long course of PEP is important for maximizing effectiveness.
  • Starting PEP as soon as possible after a potential exposure to HIV, but not more than 72 hours afterwards. The sooner PEP is started after an exposure to HIV the more likely it is to work because the drugs need to start interrupting HIV replication as soon as possible.
  • Taking extra precautions (for example, using condoms) to reduce the risk of being exposed to HIV again while taking PEP. The use of PEP is meant to reduce the risk from a single exposure to HIV and should only be used for emergencies.
Canadian PrEP guidelines