First, scientists decide what new health issue to study. Then, they carefully plan a study that will help them find out more about this new health question. The question could be about how a medicine works to treat a disease, or about how exercise combined with certain foods might help someone heal faster after a heart attack.

Once the study is organized, the scientists have a protocol in place which describes what will be done, how it will be done, and why each part of the study is necessary. If the protocol is ready, then the scientists can recruit people to participate in the study. Some studies have different phases. This means that there are different steps of the study depending on how long the health question or issue has been studied. Screening, prevention, diagnostic, and quality-of-life studies do not always have phases. And, generally, genetics clinical trials do not have phases.

Participants may be recruited into different phases of a study, or trial, depending on what the study is about. Phases allow researchers to safely understand a new issue with a step-by-step approach. While new issues are usually talked about as drugs, not all clinical trials involve the testing of new drugs. Clinical trials may test new medical procedures, analysis techniques or behavioral interventions.

Phase 1 trials are the first step in testing a new approach in people. In these studies, researchers evaluate what dose is safe, how something new should be given (by mouth, injected into a vein, or injected into the muscle), and how often. Researchers watch closely for any harmful side effects. Phase I trials usually enroll a small number of participants and take place at only a few locations. The dose of the new therapy or technique is increased a little at a time. The highest dose with an acceptable level of side effects is determined to be appropriate for further testing. Usually, a small number of healthy volunteers (between 20 and 80) are used in Phase 1 trials.

Phase 2 trials include more participants (than phase 1 trials), usually about 100-300. Phase 2 trials also involve participants who have the disease or condition that the product potentially could treat. In Phase 2 trials, researchers want to get further safety data and early information about the drug's good effects (efficacy). They then develop and refine research methods for future trials with this drug. If the Phase 2 trials indicate that the drug may be good (effective) --and the risks are considered acceptable (given the observed efficacy and the severity of the disease), the drug moves to Phase 3.

Phase 3 trials study the drug in a larger number of people with the disease (approximately 1000-3000). This phase further tests how good (effective) the product is and looks out for side effects. Sometimes it also compares how well the product works to a standard treatment, if a standard treatment is already available. A standard treatment is generally the most common and accepted way that the medical community deals with a disease. As more and more participants are tested over longer periods of time, the less common side effects are more likely to show up.

Phase 4 trials are sometimes conducted after a product is already approved and on the market to find out more about the treatment's long-term risks, benefits, and best use. Such a trial may also test the product in different populations of people, such as children.

Informed consent is a process by which people learn the important facts about a clinical trial to help them decide whether to participate. This information includes details about what is involved, such as the purpose of the study, the tests and other procedures used in the study, and the possible risks and benefits. In addition to talking with the researcher, doctor or nurse, people receive a written consent form explaining the study. People who agree to take part in the study are asked to sign the informed consent form. However, signing the form does not mean people must stay in the study. People can leave the study at any time - either before the study starts or at any time during the study or the follow-up period.

The informed consent process continues throughout the study. If new benefits, risks, or side effects are discovered during the study, the researchers must inform the participants. They may be asked to sign new consent forms if they want to stay in the study.