All clinical trials have guidelines about who can participate. Before joining a clinical trial, a volunteer must qualify for the study. The factors that allow volunteers to participate in a clinical trial are called “inclusion criteria” and the factors that disallow volunteers from participating are called “exclusion criteria.” These criteria can include age, gender, the type and stage of a disease, previous treatment history, and other medical conditions.

Some research studies seek participants with specific illnesses or conditions to be studied in a clinical trial, while others require healthy participants. It is important to note that inclusion and exclusion criteria are used to identify appropriate participants, promote participants’ safety, and ensure that researchers learn the information they need.

In a clinical trial, a volunteer is usually assigned a specific study group. Volunteers in one study group may receive an investigational treatment or study drug while other volunteers may receive a placebo or a treatment already available.

A placebo is an inactive product used to assess the experimental treatment’s effectiveness. The participant, physician, and research staff may not know which volunteer receives a placebo and which receives the active treatment. Not knowing which participants are receiving the active treatment allows the physician and research staff to objectively observe the volunteers during the study. Regardless of which treatment volunteers receive, however, the level of medical attention and care that each receives is the same.

After a study phase is complete, the data is collected to determine the drug’s effectiveness, if it is safe and if there are any side effects. Depending on the results, researchers then determine whether to stop testing or move to the next phase of study. After phase III of a study is complete, researchers decide if the results are medically important and may submit them to journals for peer-review.

Data then may be submitted to the Food and Drug Administration (FDA) for approval. If a drug is approved, pharmaceutical companies may continue to conduct studies that compare the new drug—in terms of its safety, effectiveness, and cost—to other drugs already on the market or assess a drug’s long-term effectiveness and its impact on the quality of a person’s life.