There are people who take drugs for recreational purposes and there are those who do so for financial gain.
In the current climate, many people are thinking about new or additional means to make ends meet, so they could entertain the idea of going to a clinic to be fed as-yet-unapproved drugs.
Clinical trials on humans take place to enable pharmaceutical companies to see whether or not they have any nasty side-effects from the drugs under development.
Depending on the type of product and the stage of its development, companies enrol healthy volunteers into pilot studies.
There are, however, some people who volunteer to be human guinea pigs as a full-time living.
The development of a new drug involves computer simulation and in vitro tests, followed by testing the drug on animals, mainly rodents, for toxicity.
The drug then has to be officially cleared for human testing.
During the initial clinical trial, researchers are often testing a treatment which may be in the earliest stages of its development.
Between 10 and 30 people are usually recruited for each trial.
Information gathered provides data on how the medicine is tolerated, how the body absorbs it, breaks it down and eliminates the drug, and whether any side effects have been experienced by those taking part in the trial.
The second stage of any trial involves testing the drugs on people who have a disease or condition which needs to be treated.
"People are paid to assess the safety of a drug - not if the drug works, but if it is safe to be consumed," explains anthropologist Roberto Abadie, who has been studying this career path in the US.
The industry says it has guidelines and follows regulations when it comes to professional guinea pigs.
But Mr Abadie is concerned because he doubts whether the companies are aware that some people take part in numerous trials.
"There is no central register for people who do these trials, so I'm not sure they are aware of the professionalisation," he says.
"I don't know if they know or if they care that some people do 100 trials," he adds.
Many of those who participate in drug trials are poor African Americans and poor Latinos.
"There are probably only a few thousand guinea pigs but the bulk of trials are run with these people, who derive their income from such trials," Mr Abadie says.
Individuals can earn $20,000 (£13,000) a year doing eight or 10 trials.
One trial usually lasts for about one month from the screening to the time they finish.
Former professional guinea pig Robert Helms says: "I probably never made more than $20-25,000 a year, but I knew people who were making a lot more than that. I called them workaholics."
According to Mr Abadie, some people have been known to do two trials at the same time.
"It is more than they can earn working at McDonalds, which is a reference for a low-paying job," he says.
Mr Abadie is worried about the large doses of chemicals people have in their bodies.
"Nobody knows what effect this will have when they interact with each other over 20-30 years," he says.
Mr Abadie believes the pharmaceutical industry should have concerns as well.
In 2006, a drug trial in London went badly wrong, leaving one volunteer in a coma.
"There is a big issue about liabilities if it is proven down the line that the industry had suspicions that something could go wrong, but didn't do anything because they needed the professional guinea pigs to keep coming to the trials so they could keep producing the drugs," he says.
The pharmaceutical companies stress that they take every precaution possible when it comes to the human stages of drug trials.
Mr Helms can only recall one incident when things went wrong.
"One friend back in 1996 had a very difficult time. He was very delusional, he thought the movie 12 Monkeys was actually about him," he says.
In the UK and Europe there has been new regulation over the previous few years, but not in the US.
Mr Helms wonders if enough is being done there.
"In the US the pharmaceutical industry owns the place, they are not going to change anything any time soon," he says.
"They have more political clout than they know what to do with. It is a bad situation and I don't believe it is going to change."