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Clinical Trial Glossary

Clinical trials or studies have a language of their own. It helps to understand the common terms doctors use when talking about a clinical trial before you sign up. Here are some of the words you may hear if you decide to join a trial.1

Adverse event: An undesirable side effect that affects a person during a clinical trial. The adverse event may or may not be caused by the treatment being studied.

Arm: A certain group of people in the study. Each arm of a study gets a specific treatment, or no treatment, so that results can be compared.

Baseline: Information collected at the beginning of a study on each person. This may include things like age, gender, weight, race, and test results.

Clinical trial: A medical research study in which doctors compare new treatments to existing treatments to see which works better. New drugs, devices, surgical techniques, vaccines, diet changes, exercise, education, and counseling are tested this way.

Controlled trial: A type of study that compares a new treatment to an older treatment or no treatment. The group of patients getting the older treatment or no treatment (a placebo) is called a control.

Double-blind trial: A study in which neither the doctor nor the patient knows who is receiving a treatment and who is getting a placebo. This is done to avoid any bias in the results. It is considered the strictest form of clinical trial.

Exclusion criteria: Things that prevent a person from joining in a study. This might be age, weight, gender, treatments tried before, or certain health conditions.

FDA: The U.S. Food and Drug Administration. The FDA is the government agency that approves new treatments to be sold in the U.S.

Inclusion criteria: Reasons why a person can join a study. Things that qualify a person to join a study may include age, sex, type and stage of disease, or treatments tried before. Inclusion criteria may also be called eligibility criteria.

Informed consent: The process of making sure the person joining a study understands the potential risks and benefits of that clinical trial. Informed consent also includes making sure the person understands what they will need to do to join the study and that they can quit at any time. An informed consent form outlines all of this information.

Intervention/treatment: An intervention is the treatment being studied. This may be a new drug, device, surgery, vaccine, diet change, exercise, or type of education or counseling. It can also be something else completely.

Institutional Review Board (IRB): An independent group that reviews the details of a study before it starts. This is done to protect the rights, safety, and well-being of the people who join the study. An IRB may also be called an Independent Ethics Committee (IEC).

Investigator: One of the researchers carrying out the study.

NCT number: A unique number given to each clinical trial that is registered on This is 1 way to find a study you want to learn more about.

Phase 1, Early: A stage used to gather basic information before Phase 1 begins. Humans may take what is known as a microdose of a new drug at this stage. Phase 1, Early was once known as Phase 0.

Phase 1: A stage in which a small group of volunteers takes a drug or tests a new device. Doctors then study the most common and serious side effects, how the drug is processed in the body, and what a safe dose is. The group may include 10 to 20 people or as many as 100 volunteers.

Phase 2: This stage looks closer at how well the new treatment works on a slightly larger group of people. This helps doctors learn more about the safety of the new treatment.

Phase 3: An even larger group of people use the drug or new device. At this point, several hundred to several thousand people may be trying a new treatment. The new treatment may be compared to older forms of treatment or a placebo. Doctors continue to collect information about safety and how well the new treatment works. This is the last step before a new treatment is sent to the FDA for approval to be sold in the U.S.

Phase 4: Once the FDA approves a treatment for sale in the U.S., doctors continue to track information about its safety and how well it works. This long-term phase includes the largest number of people because the drug or device is being sold on the open market.

Open-label: A study in which the doctors and patients all know what treatment is being tested and who is receiving it. This compares to a blind test, in which only the doctor knows, or a double-blind test, in which neither the doctor nor the patient knows.

Placebo: A pill, powder, liquid, device, or method designed to look the same as the treatment being studied. A placebo does not contain active ingredients or involve the treatment method being studied, and it has no treatment value. A placebo is only used to see how well a new treatment works as compared with no treatment at all.

Principal investigator: The doctor who leads the study and is responsible for the accuracy and standards of the study.

Protocol: A plan that outlines everything about the clinical trial. This includes what is being studied, how the study is organized, who can join, and how success or failure will be measured.

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