How effective is PrEP?

Two words: PrEP works. The evidence is in- guys who take PrEP consistently and correctly (every single day without any missed doses) are protected against transmission of the HIV virus regardless of their use (or not) of additional HIV prevention tools like condoms. Poz guy began using Truvada for the treatment of HIV infection in 2004, but research on its use as PrEP did not begin until 2007, and study findings were not released until November, 2010. That study, iPrEx, showed that HIV medications like Truvada could be used not just to treat EXISTING HIV infections, but also in preventing NEW HIV infections in 44% of participants. When you first see the first study results, you might think ‘Hey! That’s underwhelming.’ It’s not- we promise. Let’s break it down like this:

When looking at ALL the study participants, those who took PrEP were 44% less likely to contract HIV during the study.


When looking at ONLY study participants with MEASURABLE BLOOD CONCENTRATIONS, those who took PrEP were 92% less likely to contract HIV during the study.


When you use modelling to estimate the protection in only those study participants who took PrEP EVERY SINGLE DAY, they would be 99% less likely to contract HIV during the study.

iPrEx was a game changer: first, it showed that PrEP could work in a real world context. More importantly, though, iPrEx established that PrEP is most effective when used consistently and correctly: taken every single day as long as you’re having types of sex where HIV can be passed. Since iPrEx in 2010, there have been even more clinical studies in cis gay guys that have documented the effectiveness of PrEP, including the PROUD study in the United Kingdom, and the Ipergay study in France and Quebec. Both PROUD and Ipergay found that study participants who took PrEP were 86% less likely to contract HIV during the study than non-PrEP users. Just like the iPrEx study, the actual effectiveness of PrEP in both the PROUD and Ipergay studies would be even higher if you only looked at those study participants who took PrEP consistently and correctly.

PrEP and On-Demand Use

You might have heard about ‘on demand PrEP’ from guys you know, or read about it in the news. On-demand PrEP is also sometimes called episodic dosing, but both terms mean the same thing: On-demand PrEP means guys don’t take PrEP every day; only when know they’re having the kinds of sex that could result in HIV being passed. On-demand PrEP was first researched in a French and Canadian study called Ipergay. Ipergay not only helped establish the effectiveness of PrEP, it challenged us to think of new ways that PrEP could be prescribed and taken for HIV prevention. Ipergay didn’t only demonstrate the effectiveness of PrEP; Ipergay demonstrated that PrEP was also effective when taken ‘on demand.’ Based on the findings of Ipergay, doctors in France and the United Kingdom can now prescribe PrEP for on demand use. In on-demand PrEP use, guys take a double dose of Truvada 2-24 hours before they have sex, and one pill daily after that sexual event for two days. This might make it sound more complicated than it is, so let’s break it down here:

  • 2 – 24 hours before sex: 2 pills (double dose of PrEP)
  • 1 day after sex: 1 pill
  • 2 days after sex: 1 pill
  • No more sex? No more pills.
  • Still having sex? 1 pill every day until 2 days after you stop having sex.

While some doctors abroad are prescribing on-demand PrEP, remember that Health Canada has only approved once-daily PrEP. On-demand PrEP might be used in other countries, but so far hasn’t been recommended by Health Canada. In Canada, official PrEP guidelines are clear: PrEP should be taken every single day as long as you’re having types of sex where HIV can be passed. Ipergay was a game changer, because just like iPrEx established that PrEP effectiveness was associated with consistent and correct use, Ipergay established that there is more than one clinical pathway to consistent and correct use PrEP use. It’s really important to note that on-demand PrEP use is not yet established by multiple studies, and like we said above, hasn’t been approved by Health Canada. If your doctor recommends on-demand PrEP for you, it’s important that you know that he’s not following the guidelines provided by Health Canada. Your decision whether or not to follow on-demand dosing guidelines, though, is your own. The last thing we’ll add here on the effectiveness of PrEP was a recent study from Kaiser Permanente, one of the largest health care providers in the United States (and North America). This study was a little different, primarily because guys weren’t separated into different control groups to compare against each other- everyone was taking PrEP, and having the kinds of sex (lots of it) where HIV can be passed. So, this study is more like a really great snapshot of known PrEP users taking PrEP in “the real world.” There’s a lot this report doesn’t tell us- there’s no measuring of blood concentrations, or self-reporting, or confidence intervals, or any of that mumbo jumbo that’s super useful to epidemiologists, but just confuses most of us. In the Kaiser study, there wasn’t a single HIV transmission in any of the participants- NOT ONE! This means that in the ‘real world,’ Kaiser study, PrEP demonstrated even HIGHER levels of effectiveness than in some of the other studies! The Kaiser Permanente study also confirmed that for the majority of guys, PrEP doesn’t change condom use (that is, it neither makes them more likely to use condoms, or less likely). There are still studies under way to help us better understand the effectiveness of PrEP according to different regimens, but make no mistake: THE TIME FOR DEBATE IS OVER. PrEP WORKS.