Do you want to be a guinea pig?

ATLANTA, Georgia (CNN) -- They want to pay me for the use of my body. No, I'm not vain, nor is anyone trying to push me into prostitution. They want me (and you) to be subjects in medical studies.

A poster in my internist's waiting room asks whether I'd be interested in participating in a cholesterol study. In my gynecologist's waiting room, another poster tries to recruit ladies for studies on prenatal vitamins and on vulvar vestibulitis (you don't want to know). On my way home from work, I hear radio ads that go something like this: "Depressed? Come join our study of a new medication."

You don't have to be sick to join a study. Often, researchers are looking for healthy people.

There are advantages to joining. You can make money, sometimes hundreds of dollars or more. To a certain extent, you're getting free medical care. If you're sick, you get the chance to try a treatment so new, you can't get it from your doctor. You're also helping your fellow human beings by being a part of medical research.

But before you sign up, here are a few key questions to ask.

1. What are the side effects of the medicine or procedure being studied?

Ask about side effects of the experimental treatment compared with the side effects of your current treatment, advises the National Institutes of Health. What should you expect? Could it make you sick? Could it have long-term effects? Will the doctors in the study follow up with you long-term?

2. Who's making money off me, and will that influence how they treat me?

The doctors running the study might stand to make money if the experimental treatment works, and there are concerns that could influence what they do with you. The nightmare scenario is that they'll enroll you in the study even when the new treatment could harm you or that they'll keep the study going even though the new treatment is clearly causing dangerous side effects.

Alta Charo, a bioethicist at the University of Wisconsin, suggests that you find out whether the recruiters, doctors and nurses involved in the study are "likely to be influenced in their judgments by money or professional connections to the study sponsor."

3. Will it cost me money to be in this study?

Even if they're paying you a stipend, there might be hidden costs. Art Caplan, a bioethicist at the University of Pennsylvania, says to ask very specific questions: If you need to hire a baby sitter while you go to required appointments, who pays? Will the study sponsor pay for lost wages if the treatment makes you sick and you miss work?

4. What happened to other people who had this experimental treatment?

"Ask about the trials that came before the one you're going to do," advised Jean Burns, who has Parkinson's disease and has participated in several clinical trials. "If it's a phase three study, ask what happened to the people in phases one and two. If it's a phase one study, ask what happened in the animal studies."

The National Institutes of Health has more information about phases of studies and how clinical trials are conducted.

5. If the experimental treatment helps me, can I keep getting it after the study ends?

Don't assume you can keep taking the treatment after the study ends; you might not be able to. Ask about this in advance.

When it comes to studies, this is the bottom line: The purpose of the study is to investigate a new treatment, not to take care of you as a patient. As Charo puts it, "In a study, while you are important and the professionals will look out for you, their primary interest is in getting data. You come second."

This means you have to know your rights. The NIH has a bill of rights for study participants. Show Us the Cure, a group Burns is involved with, has a bill of rights for Parkinson's patients, but it applies to others as well.

If you're going to join a study, be prepared for disappointment. Maybe the new treatment will work, but maybe it won't. Burns spent two years taking a Parkinson's drug that turned out to be a failure' the patients on the placebo did better than the patients on the drug.