It's a different process. You're giving it to people who don't have HIV, so there is nothing to actually measure to see if they're taking it. It was originally licensed and studied as a daily pill that you take every day, to prevent you from getting HIV before you potentially get exposed to it.
Since licencing, and since some of the original studies, there have been other studies that have shown that you can actually use it in a so-called on-demand way. If you're planning on being sexually active, you can take two tablets that day, and then take a tablet a day for up to two days after you last had sex. That actually seems to work just as well as taking it every day.
One of the things we're struggling with as a program is how to measure continued use of PrEP when we know that people are using it in different ways. For the people who are saying they're using it regularly every day, that's fairly easy to monitor. For people who are using it intermittently, it's a bit more challenging.