The first known written account of a participant actually being compensated for taking part in an early version of a clinical trial occurred in late 1667 in the diaries of Samuel Pepys. During this study the subject, a homeless man, allowed himself to be injected with sheep's blood (your guess is as good as ours regarding why, but the point is that man got paid for it) [source: Junod].
Recruiting participants for clinical trials can be, well, trying. More than 90 percent of all clinical trials are delayed or are never completed because volunteers are either not appropriately matched for the study or because there are simply too few volunteers.
The first three phases require a couple flavors of recruits; people with chronic or incurable illness who may benefit from the treatment, healthy people volunteering to help advance medical research, and healthy people volunteering for the paycheck.
Anyone can apply for any study that appeals to them, but the protocol written for each clinical trial outlines who can and cannot participate during each phase -- known as inclusion and exclusion criteria. These criteria cover specifics such as age and gender, current or past medical conditions, and anything else relevant to the specific study.
During most phases of clinical trials, participants are patients seeking new treatment, prevention or cure, but during phase I, participants are healthy test subjects with no known health issues. Phase I trials offer no therapeutic benefit to the volunteer, but participation helps in advancing cures, treatments and preventative therapies for chronic or fatal conditions. It can also be a lucrative endeavor for those in good health who have the time, flexibility and inclination to take part, such as college students or those who are unemployed (or underemployed).
Medical study participants are also compensated for their time based on a per-study rate, plus a per-study determined sum based on the duration of the study, the level of hardship, and the level of risk involved. Subjects could be paid a few hundred dollars or as much as $10,000 for just one study. The best-paying studies are typically those with the strictest requirements, such as a long duration, an in-patient stay, or invasive or uncomfortable procedures [sources: McHugh, Abboud]. Participants who volunteered to stay in bed for NASA's bed-rest study, designed for researchers to learn how microgravity effects the human body, for example, got a pretty sweet deal: $18,000 for a 70-day study (which turns out to be $1,200 weekly), over a total of 15 weeks at the Johnson Space Center in Houston, Texas [source: Ziegler]. In comparison, if you worked a 40-hour-per-week minimum-wage job (at $7.25 per hour) you'd take home $267.80 each week, a financial difference not lost on repeat volunteers, known as professional lab rats or "human guinea pigs" (a term coined by George Bernard Shaw), some of whom make upwards of $50,000 annually [source: Wing, McHugh].
The higher the risk of a phase I study, the higher the compensation, but sometimes, it comes at a physical cost. In London in 2006, for example, volunteers in a phase I trial of an experimental treatment for rheumatoid arthritis were paid £2,000 for their time, but six participants suffered life-threatening organ failure and went home with long-term disabilities [source: Elliot].