Oral pre-exposure prophylaxis, or PrEP, is a way for an HIV-negative person who is at risk of HIV infection to reduce their risk of becoming infected by taking antiretroviral drugs. The daily use of oral PrEP, under the brand name Truvada, is approved by Health Canada to reduce the risk of the sexual transmission of HIV in combination with safer sex practices for people at high risk of HIV infection. Use of oral PrEP involves regular medical appointments for monitoring and support. The daily use of Truvada as PrEP is a highly effective HIV prevention strategy when used consistently and correctly. Truvada is generally safe and well-tolerated, and is available by prescription from physicians in Canada.
What Is Oral PrEP?
Oral PrEP involves the use of antiretroviral drugs by an HIV-negative person to reduce their risk of becoming infected with HIV. Oral PrEP refers to the use of a pill called Truvada, starting before someone is exposed to HIV and continuing afterwards. Truvada is also used as a treatment for HIV-positive people and contains two antiretroviral drugs: tenofovir (also called TDF) and emtricitabine (also called FTC).
Oral PrEP is intended for people at high risk of HIV infection as part of a comprehensive prevention strategy. Current guidelines recommend only the daily use of oral PrEP, however non-daily PrEP regimens are also being investigated.
How Does Oral 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 gain entry into the body, infect certain immune cells, make copies of itself (replicate) within these immune cells, then spread throughout the body.
When oral PrEP is taken consistently and correctly, antiretroviral drugs get into the bloodstream and genital and rectal tissues. The drugs work to help prevent HIV from replicating within the body's immune cells, which helps to prevent a permanent infection.
For PrEP to help stop HIV replication from happening, drug levels in the body must remain high. If pills are not taken consistently there may not be enough medication in the body to reduce the risk of HIV infection.
How Well Does Daily Oral PrEP Work?
There is evidence from randomized clinical trials (RCTs) that daily oral PrEP is a highly effective strategy to reduce the risk of the sexual transmission of HIV if taken consistently and correctly as part of a comprehensive prevention package in gay men and other men who have sex with men (MSM) and in heterosexual men and women. In addition, limited evidence from one RCT found that daily oral PrEP, when used consistently and correctly, is effective at reducing the risk of HIV transmission among people who inject drugs.
In all the clinical trials, PrEP was provided as part of a comprehensive prevention package that included regular testing and treatment for sexually transmitted infections (STIs), free condoms and ongoing behavioural counselling.
Adherence (taking medications exactly as prescribed) is crucial for oral PrEP to work. The evidence shows that higher adherence is associated with greater protection.
Before taking adherence into account, the overall risk reduction provided by a daily oral PrEP regimen in RCTs ranged from zero to 86%. All of these studies evaluated the sexual transmission risk except for one, which found a 49% overall risk reduction in people who inject drugs. The wide range of protection observed in these trials has been explained by varying levels of adherence to daily pill taking.
To demonstrate the importance of adherence, additional analyses in these trials looked at drug levels in the blood of people who were taking oral PrEP consistently compared to those who were not. These analyses found that daily oral PrEP reduced the risk of HIV transmission by between 85% and 92% among MSM and heterosexual men and women who took the drug consistently compared to those who did not. In people who inject drugs, daily oral PrEP with tenofovir alone reduced the risk of HIV transmission by 84% among people who used the drug consistently compared to those who did not.
The daily use of oral PrEP has also been evaluated in "open-label" studies, predominantly among MSM. In these types of studies, no placebo is used and all participants know they are taking PrEP and that it is effective at preventing HIV transmission. These studies support the finding that oral PrEP is highly effective at reducing HIV transmission when taken consistently and correctly. One open-label study found that the risk for HIV was reduced by 86% overall among MSM who were taking daily oral PrEP compared to those who were not. In open-label studies, adherence to daily pill taking was higher than in RCTs but was still not optimal for many people. Several studies are investigating different approaches to encourage adherence.
The daily use of Truvada as oral PrEP has been approved by Health Canada to reduce the risk of the sexual transmission of HIV in combination with safer sex practices in people at high risk for HIV infection. This approval did not include transmission through injection drug use. However, daily oral PrEP is recommended by the Centers for Disease Control and Prevention (CDC) in the United States and by the World Health Organization (WHO) to reduce the risk of HIV transmission in people at high risk through sexual exposure and in people who use injection drugs.
Does Intermittent PrEP Work?
Intermittent, or on-demand, PrEP may reduce the risk of HIV transmission among MSM who have frequent condomless sex but the evidence is limited. One RCT evaluated the use of intermittent PrEP among MSM but no studies have been conducted in other populations.
In the one trial, MSM took an initial dose of two Truvada pills (at the same time) two to 24 hours before sex, another pill 24 hours after the initial dose, and a fourth pill 24 hours later. This strategy reduced the risk of HIV infection by 86%. However, men in the study reported having frequent sex and taking an average of four pills per week. This frequent dosing may be important to maintain high levels of drug in the body. It is unclear how well an intermittent PrEP strategy would work for gay men who have less frequent sex, or for other populations such as women or people who inject drugs.
Non-daily use of oral PrEP is not approved by Health Canada; however, intermittent PrEP can be prescribed "off label" by physicians in Canada. The European AIDS Clinical Society guidelines endorse the use of intermittent PrEP only for MSM who have frequent condomless sex.
Does Oral PrEP Work as Well for Women as for Men?
Evidence from RCTs suggests that oral PrEP is as effective for women as it is for men when used consistently and correctly, but adherence may be more important for women.
There were initial concerns that PrEP may not work for women because two RCTs did not find a reduced risk of HIV in heterosexual women taking daily oral PrEP. However, adherence was very low in these studies with only a small proportion of women taking PrEP daily.
There is some evidence showing that Truvada takes longer to reach maximum drug levels in vaginal tissues compared to rectal tissues, and that drug levels are lower in vaginal tissues. This suggests that daily dosing of oral PrEP may be more important for women having vaginal sex to maintain sufficient drug levels to help prevent HIV infection.