Human medical test subject is in that category of jobs, along with sperm donor and medicinal pot grower, that kind of seem like free money. Sure, you can't get rich doing it, but it's there if you should ever need it. Take some pills, fill out some forms, maybe accidentally gain X-ray vision or something.
But, as with all things, this job is a way bigger deal than you thought. For instance ...
Get in Line, Because Everybody Wants to Do It
So you see an ad in the paper offering to pay a couple of hundred bucks to let yourself be injected with some experimental drug. You'd have to assume there isn't exactly going to be a line around the block to get in. This stuff is for the desperate and almost homeless, right?
"Just one more shot, sir, and you can get back to sleeping in storm drains."
Nope. Getting into a clinical trial can be just as hard as getting a real job, partly because you are going up against people who actually do clinical trials as their real job. In the business, they're called guinea piggers (except at Christmas dinner, when they presumably say they're waiters or something), and their numbers are in the thousands. This means that more often than not, applying for a trial forces you to compete against a gaggle of people whose resumes list their special skills as "metabolizing drugs and disregarding personal health for money."
These are the sort of folks who travel around the country doing the clinical trial circuit, calmly signing up for all kinds of research -- including tests that require them to spend weeks in a lab. They also follow a strict dietary regimen and abstain from alcohol and any drugs that could create a reaction with whatever mystery chemical they're ingesting next. Though even then, these uber-subjects might not qualify for reasons as obscure as, say, being the wrong size.
"Sadly, our shrinking potion has rendered you ineligible to test the antidote."