One way to earn a little extra cash is by volunteering as a guinea pig. There are many ranges of guinea pigs, from those who volunteer to undergo dangerous procedures all the way down to paid taste testers. Many people wonder how to become a guinea pig and how much income you can earn from it. Admittedly, this is not my area of expertise, but I'm glad to share what I do know.
Craigslist Yes, Craigslist is full of interesting things, sometimes they are legit, sometimes they are not, but this is your best source for finding paid research gigs of any scale. The most important things to always remember with any ad, be it Craigslist or elsewhere, is never to give or receive money from anyone you don't know, know what you're getting into upfront, and check credentials. If you go to your city's Craigslist, look under "Jobs" and scroll to the bottom where you see "[ETC]." A lot of the offers on this page will be scams, so be vigilant. However, a lot of reputable research companies to use Craigslist because the ads are cheap and it's hard to find any other online outlet.
Universities I am personally fortunate to live in the Atlanta area close to Emory, which is well known for medical and scientific advances. Emory has its own guinea pig service. Check with any universities near you. You may have to really dig on their websites to find it, or they may be on completely different sites. I was lucky enough to be in Piedmont Park at the time when a bunch of students were handing out guinea pig flyers for Emory (btw, NEVER refer to it as guinea pigging to anyone official).
Medical Research Institutes These are best for clinical drug trials. Again, these might be hard to track down. You can certainly use a search engine to find them, but again, don't send or receive any money, don't do anything that makes you uncomfortable, ALWAYS research credentials. Also, try paying attention to radio commercials. Radio is still a very frequently used method of advertising for medical research. These studies are usually well paid, but in return, a lot is expected. The qualification process sometimes takes hours. If you do qualify, you may be expected to take certain medications at very inconvenient times, keep extremely detailed journals, call and check in with the institute at inconvenient times, etc. It's usually nothing excruciating, but several things that are kind of annoying.
Taste Tests These can also be difficult to find. There are a few national, legit companies such as Contract Testing and Focus on Food, but there are also smaller, local research companies that may be a little harder to track down. The least amount of money I've ever accepted has been $35 and the most has been $200. They often pay cash. The only problem with taste tests is usually you either can't participate in them very often because the research companies won't accept people who've participated in studies within a designated time frame, or you have to be comfortable with lying in order to participate. I do not endorse lying to research companies. Such behavior may result in horrible consequences such as the McRib sandwich.
Blood/Plasma "Donations" This isn't truly guinea pigging because there's really no research involved, but some clinics will pay for blood donations, depending on your blood type, or plasma donations. Google is your friend for finding these in your area. Don't be surprised if they're in the more rundown parts of town.
What can I expect to earn?
Unfortunately, I can't really tell you. It depends on what you're testing and how often. As previously stated, the most I've ever earned for one study has been $200 and I usually only do taste tests. I usually only participate in taste tests, at most, every 2-3 months. This is mostly due to research companies' standards. I personally wouldn't say I make more than about $500 a year on it, but again, I shy away from the higher dollar tests that may involve an element of danger.
Anyone embarking on a journey to earn money through guinea pigging does so at her/his own risk. I do not endorse putting your life on the line or doing anything to endanger yourself for extra cash, but it's your life and I assume you're a grownup. This is strictly so you can't sue me by saying I told you to do all this. :P
I can't stress this enough: ALWAYS KNOW WHAT YOU'RE GETTING INTO!
Tune in next Tuesday for more tips, advice, or general blabla.