Let's talk about Pre-Exposure Prophylaxis (PrEP). When an HIV- negative individual takes HIV meds (Truvada) in order to prevent HIV, we call it Pre-Exposure Prophylaxis. This biomedical intervention can be used by people in serodiscordant relationships (one person is HIV-positive and the other is not), by men who have sex with men at high risk, and even serodiscordant couples who are trying to have a baby.

You probably have questions about PrEP, I know I do. I'm going to share some resources so we can all get good answers. PrEP is still a relatively new tool for HIV prevention and we are still learning how to make it work best for different types of people. There are still lots of questions. But we do know that when taken consistently and correctly PrEP greatly reduces a person's risk of HIV infection.

What's the science?

Research studies comparing PrEP methods have been done in Asia, Africa, North America (including the U.S.) and South America. There are some demonstration projects going on now to see how PrEP works in the real world for real people in the U.S. and other places. If you want a little more detail on the studies you can check out Project Inform's highlights here.

Men who have sex with men and transwomen

So far there has been one major study that tested the effectiveness of PrEP for men who have sex with men (MSM) and transwomen. The iPrEx study included almost 2500 MSM and transwomen in six countries. Participants were assigned to one of two groups: Truvada with comprehensive HIVcounseling, condoms, STI testing and treatment with HIV testing and a group with the same supports and treatment but who were given a placebo (sugar pill). This study was double-blinded, meaning neither the researchers or the participants knew who received the Truvada or the sugar pill. Results showed that adherence to the daily Truvada regimen was the single most important factor for effectiveness of PrEP. Those in the Truvada group who took the drug regularly enough to have measurable amounts of the drug in their blood were protected up to 92% over the placebo group. But those in the Truvada group who did not take the drug as regularly were only protected by 42%. Daily use (as measured in drug blood levels) protected up to 99%.

Heterosexual men and women

Two studies with heterosexual men and women in African countries showed similar results for those who had high adherence to the daily regimen on Truvada: up to 90% protection. These studies were with serodiscordant couples. More than 4500 hetereosexual adults participated in the Partners PrEP study and 1200 in the TDF2 study.
Studies that tested PrEP effectiveness in women were not as successful as those on serodiscordant heterosexual couples. The Fem-PrEP study with 2100 African women was stopped early because there was no difference in protection between the Truvada group and the placebo group. Later follow-up showed that the drug blood levels in most of the women did not indicate use of the drug as directed. Adherence was actually very low, despite women reporting taking the medicine as directed. Another study of women, VOICE,  had similar issues of adherence. 
There are several possible reasons for the women agreeing to be in the study and reporting adherence to the drugs, even while not complying with the regimen, including: free access to health care, access to Truvada for HIV-positive relatives, financial compensation and other benefits of study participation. We also know that research participants may report behavior they think is the desired answer in order to avoid embarrassment, make the researchers feel good, forgetfulness, or reasons other than dishonesty. This is why whenever possible, researchers use biological markers, like drug levels in blood, to determine the real adherence levels in a drug trial.

Intravenous drug users

One study has been completed testing PrEP effectiveness for intravenous drug users (IDU) in Bangkok, Thailand. These results are complicated because the IDU participants may also have been having unprotected high risk sex while in the trial, so it is hard to isolate if Truvada works for IDU HIV exposure. In this study, 2400 self-identified IDU were assigned either to Truvada or a placebo, with access to support like risk counseling, condoms, HIV testing, etc. Overall there was a 49% reduction in HIV infection in the Truvada group, with 74% reduction in the group that had directly-observed treatment (they took the Truvada in front of a researcher on a daily basis). 

Other resources

So that's some of the PrEP science. There's a lot more coming out all the time, including data on risk behaviors, adherence, effectiveness in real life, etc. Stay tuned for more on PrEP but for now here are a few resources to learn more.
My PrEP Experience - firsthand accounts of people who are on PrEP
Project Inform's PrEP videos - videos on making the decision to be on PrEP, PrEP in serodiscordant relationships and more.
My Life on PrEP - series from Frontiers L.A.
C.D.C.'s Q & A on PrEP clinical research