I once participated in a drug study. I went to a facility and was given the medication. I then had a form which I used to record any side effects I had.
Our group leader later said that since all of us in the study had MS, that the study was also known as a cross-sectional study. I have never heard of that before. She said it referred to the fact that we all have an illness, and we were being monitored over a period of time.
All I know is that in the end, I got paid for my participation and I found the experience to be rewarding. Not only did I receive treatment, but I helped others by recording my experiences with the drug.