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Understanding Clinical Trials

Excerpted from, a service of the National Institutes of Health

What is a clinical trial?

A clinical trial is a research study to answer specific questions about vaccines, new therapies or new ways of using known treatments. Clinical trials (also called medical research and/or research studies) are used to determine whether new drugs or treatments are both safe and effective. Carefully conducted clinical trials are the fastest and safest way to find treatments that work in people.

Why participate in a clinical trial?

Participants in clinical trials play a more active role in their own healthcare, gain access to new research treatments before they are widely available and help others by contributing to medical research.

What is a protocol?

A protocol is a study plan on which all clinical trials are based. The plan is carefully designed to safeguard the health of the participants as well as answer specific research questions. A protocol describes what types of people may participate in the trial; the schedule of tests, procedures, medications, and dosages; and the length of the study.

What are the different types of clinical trials?

Treatment trials test new treatments, new combinations of drugs or new approaches to surgery or radiation therapy.

Prevention trials look for better ways to prevent disease from returning. These approaches may include medicines, vitamins, vaccines, minerals or lifestyle changes.

Screening trials test the best way to detect certain diseases or health conditions.

Quality of life trials (or Supportive Care trials) explore ways to improve comfort and the quality of life for individuals with a chronic illness.

What are the phases of clinical trials?

Clinical trials are conducted in phases. The trials at each phase have a different purpose and help scientists answer different questions:

In Phase I trials, researchers test a new drug or treatment in a small group of people (20-80) for the first time to evaluate its safety, determine a safe dosage range and identify side effects.

In Phase II trials, the study drug or treatment is given to a larger group of people (100-300) to see if it is effective and to further evaluate its safety.

In Phase III trials, the study drug or treatment is given to large groups of people (1,000-3,000) to confirm its effectiveness, monitor side effects, compare it to commonly used treatments and collect information that will allow the drug or treatment to be used safely.

In Phase IV trials, post-marketing studies delineate additional information including the drug's risks, benefits and optimal use.

What is informed consent?

Informed consent is the process of learning the key facts about a clinical trial before deciding whether or not to participate. It is also a continuing process throughout the study to provide information for participants. To help someone decide whether or not to participate, the doctors and nurses involved in the trial explain the details of the study. Informed consent is not a contract and the participant can withdraw from a trial at any time.

What is an Institutional Review Board?
  1. A committee of physicians, statisticians, researchers, community advocates and others that ensures that a clinical trial is ethical and that the rights of the study participants are protected. All clinical trials in the U.S. must be approved by an IRB before the study begins.
  2. Every institution that conducts or supports biomedical or behavioral research involving human participants must, by federal regulation, have an Institutional Review Board that initially approves and periodically reviews the research in order to protect the rights of human participants.