These are called inclusion/exclusion criteria, and not only protect the participants from harm but also allow the researchers to be sure that the studies they do provide reliable data on the safety and efficacy of the drug. Each person who takes part in a drug study must go through an informed consent process. The potential participant will be given facts about the study, and is allowed to make a good decision regarding whether or not to participate. Even if the participant agrees to join a study, the participant can quit the study at any time. Participants may join a study for many reasons: they may want to try an experimental treatment, or want to help in medical research. Often time's participants will receive some sort of financial benefit for participating in studies, usually based upon the risk or requirements of the study. The funding for clinical trials...

Most commonly the studies are performed and paid for by pharmaceutical companies, but may also be paid for by federal agencies like the National Institutes of Health (NIH), the Department of Defense (DOD) or the Department of Veteran's Affairs (VA). In some cases, drugs may be made available for treatment of certain conditions before they actually receive final approval from the FDA. These drugs are called investigational, and usually used to treat a life-threatening illness for which early trials appear promising and in cases where other treatments have not been helpful.
References

Data obtained from the internet at http://clinicaltrials.gov/ct2/info/new

No Author Listed, (2008), Protecting the Right of Human Participants. Accessed via the NIH website at http://phrp.nihtraining.com/users/login.php