If you want to be included in a study, it is fairly easy to adjust to give yourself some symptoms. I used to eat eggs every morning. When I stopped - boom - 50 point drop in cholesterol. It won't work for everything, but diet and pharmaceuticals could give you a whole lot of physical symptoms if you were so inclined. Disease symptoms can be cured. Probably at least as easy to induce them. There are lots of ways people sell their bodies for money. Inducing temporary disease symptoms surely isn't the worst.

This is especially true for antidepressants because some are only effective on more severe cases (eg Zoloft); self-selection will yield a body of faux-depressed and mildly depressed people on whom the drug has no result.

EDIT: Apparently I was thinking of a different drug.