They can, and sometimes do, go awry (see "A dangerous occupation" lower down in the article.
SHOOT: I did this for a spell. It's not something I'd want my kids to be doing.
The 32-year-old former hotel manager is just one of tens of thousands of people around the world who make a living out of participating in clinical trials (see "Guinea pigs 'R us").
While the money can be good, this is potentially dangerous work.