CATIE

HIV pre-exposure prophylaxis (PrEP)

Summary

Pre-exposure prophylaxis, or PrEP, is a highly effective HIV prevention strategy. People who are HIV negative can use it to lower their chance of getting HIV. Using PrEP involves taking medication and having regular medical appointments for monitoring and support. When it is taken as prescribed, the risk of HIV transmission is extremely low. PrEP is safe and usually well tolerated and is available by prescription. In Canada, PrEP is available in pill form (also known as oral PrEP) and as a long-acting injection given by a healthcare provider.

What is PrEP?

PrEP is an HIV prevention method that can be used by people who are HIV negative and at ongoing risk for HIV. It is a highly effective way to prevent HIV transmission when used consistently and correctly. It involves taking antiretroviral medications, which are the same drugs used to treat people with HIV. Taking PrEP requires a prescription from a healthcare provider and attending regular medical appointments for monitoring and support. PrEP is available in pill form (also called oral PrEP) or as a long-acting injection.

PrEP is not the same thing as post-exposure prophylaxis (PEP). PrEP is taken regularly before and after potential exposures to HIV, whereas PEP is taken for 28 days after a single potential exposure. 

What types of PrEP are available in Canada and who are they recommended for?

There are three types of PrEP approved by Health Canada, including two different pill formulations and one long-acting injectable option. Both PrEP pills contain two drugs. Injectable PrEP contains just one drug.

In 2016, a pill containing the drugs tenofovir disoproxil fumarate (also called TDF) and emtricitabine (also called FTC) was the first drug combination to be approved by Health Canada for use as PrEP. It is sold under the brand name Truvada and generic versions are also available. This form of PrEP can be taken daily or on-demand (only on days before and after having sex). Daily PrEP is recommended as an option for anyone who may be at risk for HIV through sex or from sharing injection drug use equipment. On-demand PrEP (also known as PrEP 2-1-1) is recommended as an option to prevent getting HIV through sex for cisgender (cis) men — including gay, bisexual and other men who have sex with men (gbMSM) and heterosexual men — and for transgender (trans) women. 

In 2020, a second drug combination was also approved for use as PrEP. This pill contains tenofovir alafenamide (also called TAF) and FTC. It is available under the brand name Descovy. It is taken daily and is recommended as an option for anyone who may be at risk for HIV through sex or from sharing injection drug use equipment.

Most recently, in 2024, long-acting injectable PrEP with the drug cabotegravir (also called CAB) was approved for use in Canada. It is available under the brand name Apretude. This form of PrEP is recommended as an option for anyone who may be at risk for HIV through sex. 

What are the possible ways to take PrEP?

Daily PrEP 

A person takes one pill every day. 

On-demand PrEP 

A person takes pills only on days before and after having sex. The term PrEP 2-1-1 refers to the dosing schedule, which involves taking:

  • 2 pills between 2 and 24 hours before sex
  • 1 pill 24 hours after the first dose
  • 1 pill another 24 hours later

If a person continues to have sex, they should continue to take a pill every 24 hours until two days after the last time they have sex. 

When a person anticipates having sex again, if it has been less than a week since their last dose they can take just one pill as their starting dose (between 2 and 24 hours before they have sex). If it has been more than a week since their last dose, they should take a starting dose of two pills.

Long-acting injectable PrEP 

A person receives long-acting injections from a healthcare provider monthly for the first two months, then every two months after that. A person can choose to take CAB in pill form for 28 days before they start getting injections to make sure they tolerate the drug. This lead-in period is optional because studies show that CAB injections are usually very well tolerated. 

How does PrEP work to help prevent HIV?

PrEP interferes with the pathways that HIV uses to cause a permanent infection. 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 PrEP is taken consistently and correctly, antiretroviral medication gets into the bloodstream and genital and rectal tissues. The medication works to prevent HIV from replicating within the body’s immune cells, which helps to prevent a permanent infection.

What is the evidence for the effectiveness of each form of PrEP?

Daily PrEP

A large body of evidence shows that daily PrEP using TDF + FTC is highly effective at reducing the risk of HIV transmission. This PrEP regimen was initially proven effective in randomized controlled trials (RCTs) conducted in gbMSM, trans women and heterosexual men and women. In these studies, the overall reduction in HIV risk ranged from zero to 86%, but these calculations included data from people who did not take PrEP as prescribed. Additional analyses showed that the use of PrEP (determined by detectable drug in the blood) reduced the risk of sexual HIV transmission by between 85% and 92%. These data highlight the fact that adherence (taking medication exactly as prescribed) is crucial for PrEP to be effective.

Among all the studies and several million people now using PrEP globally (including people of all genders and sexual orientations), there have been very few documented cases of sexual HIV transmission in people who are taking it as prescribed. Based on all of this evidence, daily PrEP with TDF + FTC appears to prevent nearly 100% of sexual HIV transmissions when it is taken consistently and correctly.

No study has looked at daily PrEP using TDF + FTC among people who inject drugs. However, one RCT looked at daily PrEP using TDF alone in this population. That study found a 49% overall risk reduction; however, HIV risk was reduced by 84% among people who used TDF consistently compared with those who did not. Although TDF alone is not recommended as PrEP for any population, this study provides evidence to support the recommendation that daily PrEP can be used by people who may be at risk for HIV through sharing injection drug use equipment.

Daily PrEP with TAF + FTC has been shown to be effective in one RCT with gbMSM and a small number of trans women. This study found it to be just as effective as daily TDF + FTC when taken consistently, as prescribed. All HIV infections in the study were among people who did not take their medication consistently and correctly.

On-demand PrEP

On-demand PrEP has been studied only with TDF + FTC, among gbMSM and a small number of trans women. One RCT found an 86% reduced risk of HIV infection among gbMSM taking on-demand PrEP compared with those taking a placebo pill. Since then, several studies among gbMSM and trans women have shown that on-demand PrEP is as effective as daily PrEP when it is taken correctly. In all of the studies about on-demand PrEP, all HIV infections were among people who had stopped taking PrEP or who did not take it consistently and correctly.

Long-acting injectable PrEP

Two RCTs have shown that injectable PrEP with CAB is highly effective at preventing HIV transmission through sex. One of these studies was among gbMSM and trans women and the other was among heterosexual women. Both studies found that injectable CAB was more effective than daily TDF + FTC at preventing HIV infection. In both studies, the higher number of infections in the daily oral PrEP group was linked to non-adherence, suggesting that injectable PrEP may be more effective for some people because it eliminates the need to remember to take a pill every day.

What is involved in taking PrEP besides taking medication?

PrEP is part of a comprehensive HIV prevention strategy that involves routine medical appointments and should include access to other supports and services such as free condoms and safer injecting supplies and referrals to other services (e.g., mental health services, opioid substitution therapy).

The first step is to make sure a person is HIV negative before they start PrEP. They should also be tested for sexually transmitted infections (STIs) and hepatitis A, B and C and they may need some additional tests depending on the type of PrEP that they would like to take.

Once a person starts taking PrEP they must attend regular medical appointments, typically once after the first month on PrEP and then every three to four months thereafter. These visits are necessary so that the person can be tested for HIV and other STIs, be monitored for drug side effects and receive ongoing adherence and risk-reduction counselling.

Who should take PrEP?

PrEP is for people who are HIV negative and at risk of getting HIV through sex, sharing injection drug use equipment or both. While there is no perfect formula for determining PrEP eligibility, certain factors can help to identify people who could benefit from PrEP.

People may want to consider PrEP if they:

  • sometimes have vaginal or anal sex without using a condom and don’t know the HIV status of one or more of their sex partners
  • have a sex partner who is HIV positive and not on successful treatment
  • inject drugs and sometimes share needles or other equipment
  • have ever tested positive for an STI or hepatitis C
  • have ever used post-exposure prophylaxis (PEP)

These criteria can help identify people who may benefit from PrEP, but they should not be used to deny access to someone who asks for PrEP. It is important to recognize that some people feel uncomfortable disclosing their sexual or drug use behaviours to healthcare or other service providers.

How can people access PrEP?

A person who wants to take PrEP needs to get a prescription from a healthcare provider. A doctor or nurse practitioner can prescribe PrEP. In some regions, other healthcare providers such as pharmacists may also be able to prescribe it. A person may be able to access PrEP from a sexual health clinic, a specialized HIV clinic, their primary healthcare provider or, in some regions, a pharmacy. There are also online PrEP clinics available where PrEP prescribing and follow-up are done online and medications can be delivered.

PrEP with TDF + FTC is on most provincial, territorial and private drug plans in Canada. This means that most people will not need to pay the full price out of pocket. PrEP using TAF + FTC and injectable CAB are both not currently as widely included on public and private drug plans. They are also not currently available in generic formulations, so they are more expensive than TDF + FTC if paid for out of pocket. 

What are some of the safety concerns associated with taking PrEP?

Side effects

Although the drugs used in PrEP are generally well tolerated, they are still capable of causing side effects. 

In clinical trials the side effects from taking PrEP in pill form were generally mild and temporary, and they affected only between 1% and 10% of participants. Some of the possible side effects include nausea, vomiting, diarrhea, headache and dizziness. PrEP with TDF + FTC has been associated with more concerning toxicities in a small number of people, such as small decreases in kidney, bone and, rarely, liver health. Promisingly, these changes were reversible after stopping PrEP. The TAF + FTC formulation is generally not associated with the kidney and bone risks of TDF + FTC.

The most common side effect of injectable PrEP with CAB is mild to moderate pain or tenderness at the injection site; however, this tends to decrease with subsequent injections. Other possible side effects include headache, diarrhea and feeling hot.

Drug resistance

Drug resistance can develop if someone is HIV positive when they start taking PrEP or becomes HIV positive while on PrEP. If this happens, the drugs in PrEP may not work to treat the person’s HIV, which can limit the person’s options for HIV treatment. To avoid resistance, it is important to confirm that a person is HIV negative before they start PrEP and to make sure that they continue to have regular HIV testing while using it.

If a person tests positive for HIV while on PrEP, they should stop taking PrEP right away to reduce the risk of developing resistance. They should immediately be offered HIV treatment.

How long after a person starts taking PrEP can they rely on it to prevent HIV?

PrEP does not prevent HIV immediately after a person starts taking it. People who are taking daily PrEP or long-acting injectable PrEP should wait seven days from when they take their first pill or get their first injection before relying on it to prevent HIV. People who take PrEP on-demand should take their first dose of two pills between 2 and 24 hours before having sex. 

Can PrEP be stopped and restarted safely?

A person can stop using PrEP and restart it again, depending on their risk for HIV, which may change over time. If a person wants to stop taking PrEP, or restart after a period of not taking it, they should talk to a healthcare provider about how to do so safely. Before restarting PrEP, a person should be tested for HIV if there has been any possible HIV exposure since they last took PrEP. 

Are there other types of PrEP? 

Studies have been done, or are underway, looking at various other types of PrEP in addition to the three types that are currently available in Canada. 

A twice-yearly injection with the drug lenacapavir was found to be effective at preventing HIV infection in two large trials. This regimen has not been approved by Health Canada, but it may be in the future. 

An intravaginal ring that can be used by cis women to prevent HIV has been approved for use in some African countries. However, it has not been approved in any high-income countries because the level of protection is much lower than that provided by PrEP taken in pill form or by injection.

Other types of PrEP, including other types of PrEP pills, vaginal or rectal gels, and implants, are currently in experimental stages. These forms of PrEP have not been approved for use by any regulatory agency in the world, and we do not expect them to be available for use in Canada in the near future. 

Client resources

PrEP to prevent HIV: Your questions answered – brochure

Is PrEP right for me? – pocket card

How does PrEP prevent HIV? – video

Service provider resources

Previewing Canada’s draft guideline for HIV PrEP and PEP – webinar

What’s the 411 on PrEP 2-1-1? On-demand PrEP for preventing HIV – Prevention in Focus

Worth a shot: Injectable PrEP for HIV prevention – Prevention in Focus

La prophylaxie préexposition au virus de l'immunodéficience humaine : Guide pour les professionnels de la santé du Québec – Ministère de la Santé et des Services sociaux du Québec (French only)

Guidance for the use of pre-exposure prophylaxis (PrEP) for the prevention of HIV acquisition in British Columbia – BC Centre for Excellence in HIV/AIDS

2021 Alberta HIV pre-exposure prophylaxis (PrEP) guidelines – Alberta Health Services

Pre-exposure prophylaxis: Guideline review for primary care practitioners in Saskatchewan – Saskatchewan HIV Collaborative

References

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  12. van den Elshout MA, Wijstma ES, Boyd A et al. Sexual behaviour and incidence of sexually transmitted infections among men who have sex with men (MSM) using daily and event-driven pre-exposure prophylaxis (PrEP): four-year follow-up of the Amsterdam PrEP (AMPrEP) demonstration project cohort. PLoS Medicine. 2024 May 8;21(5):e1004328.
  13. Landovitz RJ, Donnell D, Clement ME et al. Cabotegravir for HIV prevention in cisgender men and transgender women. New England Journal of Medicine. 2021 Aug 12;385(7):595-608.
  14. Delany-Moretlwe S, Hughes JP, Bock P et al. Cabotegravir for the prevention of HIV-1 in women: results from HPTN 084, a phase 3, randomised clinical trial. The Lancet. 2022;399(10337):1779-89.

Authors: Arkell C, Harrigan C.

Updated: 2025