There's actually a community of people who make a living doing this, travelling the nation from lab to lab. The amount one is paid is based on may factors, like length and number of times one is 'stuck' (blood drawn or meds injected). In the 90's, I spoke with one guy that made over $25,000 a year, inclding one study that nabbed him $5,000 for 3 weeks of eating ice cream (they drew his blood 10 times a day).

It's not a bad way to earn a little cash. I'd be weary of drug tests though. They really can't guarantee it won't be harmful to humans.