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HIV Pre-Exposure Prophylaxis (PrEP)

How PrEP works

PrEP is a medication, that when taken correctly at a sufficient dose, blocks HIV from getting into your body if you are exposed.

The medication used for PrEP is a tablet which contains tenofovir and emtricitabine (drugs commonly used to treat HIV positive patients).


It is known best as brand name Truvada but most PrEP tablets are now generic (less expensive!) and works exactly the same. There is also brand name Descovy (or generic equivalent) which you may have heard more recently being approved for PrEP in Canada. 

Many other PrEP delivery methods such as injectables and implants are also currently being researched. Exciting!

Taking PrEP

In research and PrEP clinical trials, PrEP has been used in two different ways:

  • daily dosing - taken regularly (one tablet per day)

  • on-demand / "disco dosing" - only taken when needed (two tablets two to 24 hours before sex, one tablet 24 hours after sex and a further tablet 48 hours after sex).

Both of these methods have been shown to be very effective in preventing HIV transmission, although on-demand dosing has only been studied in gay and bisexual men, and is not yet approved in Health Canada for this reason. It would be an off-label use. 

Daily PrEP is recommended for all trans people currently taking hormone treatment as there is limited data to support other dosing options.

Daily dosing is recommended for cis-women who need to take PrEP every day for seven days to be protected against HIV.

Taking PrEP safely

If you don't take it correctly, it may not work.

The drugs used in PrEP are the same drugs that are prescribed to thousands of people living with HIV every year. They’re very safe and serious side effects are very rare. 

A few people experience nausea, headaches or tiredness and, very rarely, the medication can affect kidney function. As a precaution, people taking PrEP have regular kidney and liver function tests.

Condoms are still encouraged! Even though PrEP is highly effective at preventing HIV, it does not protect you from other STIs or pregnancy. 

Routine sexual health and STI screening is also important, and when taking PrEP it is important to be tested for infections every 3 months, especially if you have been sexually active. 

If you'd like to discuss PrEP further, or to get started, click below to book an appointment!


More information regarding PrEP can also be found at CATIE / PREP.

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