Understanding Clinical Trials

A clinical trial is a research study involving a select group of people who have volunteered to try an intervention — such as a drug or other therapy, vaccine, behavior change regimen or procedure — under close medical monitoring. The study answers researchers’ questions about the safety and/or effectiveness of the intervention. Before reaching the clinical trial stage, new interventions typically have been studied in the laboratory (in nonclinical studies) and in animals (in preclinical studies).

Below are frequently asked questions about clinical trials and observational studies. To learn about specific studies that are ongoing at Fred Hutch, visit the Clinical Trials page, where you can search by disease or other criteria and view all studies.

What are clinical trial phases?

Clinical trials for treatments and vaccines typically have four phases, each of which is designed to answer different research questions. The phases are:

  • Phase 1: Safety
  • Phase 2: Efficacy
  • Phase 3: Comparison
  • Phase 4: Post-Marketing

If an intervention is successful in one phase, it may move on to the next phase. Each successive phase typically has a greater number of participants. Sometimes two phases are combined. In a Phase 3 study, each participant is randomly assigned to one of two groups. Only one group will receive the new intervention being studied; the other will receive the standard treatment for that disease.

Observational studies, such as prevention and quality-of-life studies, do not include an intervention, so they do not follow these phases.

Who participates in clinical trials and observational studies?

Depending on the clinical trial or observational study, the participants may be people who have been diagnosed with a particular disease, are survivors of a disease, are at heightened risk of a disease or are simply healthy volunteers. Some people participate in a clinical trial because there are no standard treatments available to them or because other treatments they have tried haven’t worked. Others participate for altruistic reasons, such as to advance medical knowledge. People who volunteer for lifestyle and prevention studies may be genuinely interested in learning ways to improve their health and reduce their risk of disease.

How do I know if I am eligible to participate?

The descriptions of clinical trials and observational studies clearly spell out who is potentially eligible. Criteria may include age, gender, ethnicity, type and stage of disease, or other characteristics. Many studies also spell out medical conditions that may exclude a person from eligibility.

Are study participants paid?

Some studies pay volunteers for their participation — particularly lifestyle studies and studies that involve healthy volunteers. This will be spelled out clearly in the study description. Depending on the number of clinic visits and the duration of the study, volunteers may be paid up to a few hundred dollars upon completion of the activities.

What are the four phases of clinical trials?

Clinical trials advance through four phases to test a treatment, find the appropriate dosage, and look for side effects. If, after the first three phases, researchers find a drug or other intervention to be safe and effective, the FDA approves it for clinical use and continues to monitor its effects.

Clinical trials of drugs are usually described based on their phase. The FDA typically requires Phase I, II, and III trials to be conducted to determine if the drug can be approved for use.

  • A Phase I trial tests an experimental treatment on a small group of often healthy people (20 to 80) to judge its safety and side effects and to find the correct drug dosage.
  • A Phase II trial uses more people (100 to 300). While the emphasis in Phase I is on safety, the emphasis in Phase II is on effectiveness. This phase aims to obtain preliminary data on whether the drug works in people who have a certain disease or condition. These trials also continue to study safety, including short-term side effects. This phase can last several years.
  • A Phase III trial gathers more information about safety and effectiveness, studying different populations and different dosages, using the drug in combination with other drugs. The number of subjects usually ranges from several hundred to about 3,000 people. If the FDA agrees that the trial results are positive, it will approve the experimental drug or device.
  • A Phase IV trial for drugs or devices takes place after the FDA approves their use. A device or drug's effectiveness and safety are monitored in large, diverse populations. Sometimes, the side effects of a drug may not become clear until more people have taken it over a longer period of time.