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Clinical Trials Glossary

Blinded (or Masked) Study
In a blinded or masked study, participants do not know whether they are in the clinical trial’s experimental group or control group. Participants in the experimental group get the medications or treatment being tested, while the participants in the control group get a standard treatment or no treatment. The study staff knows which participants are in each group.

Control Group
In many clinical trials, one group of subjects will be given an experimental drug and another group, called the control group, will be given either a standard treatment for the illness or a placebo.

Double-Blind (Double-Masked) Study
In a double-blind or double-masked study, neither the participants nor the study staff know who is receiving the experimental drug and who is getting a standard treatment or placebo.

Healthy Volunteer
A healthy volunteer is a clinical trial participant who has no known significant health problems.

Inclusion/Exclusion Criteria
Inclusion criteria are factors that allow someone to participate in a clinical trial. Exclusion criteria are factors that prohibit someone from participating in a clinical trial.

Informed Consent
Informed consent means that a study’s participant has been told the purpose, risks and benefits of the clinical trial research, including what their role will be and how the trial will work.

Institutional Review Board (IRB)
An Institutional Review Board (IRB) is a committee of scientists, healthcare professionals and community members. The group reviews every new clinical trial to evaluate its benefits and risks and to make sure patient rights are protected.

Patient Volunteer
A patient volunteer is a person with a known health problem who participates in a clinical trial to better understand, diagnose, treat or cure that disease or condition.

Phases of Clinical Trials
Clinical trials are conducted in “phases.” Each phase has a different purpose and helps researchers answer different questions.

  • Phase 1 trials: In a phase 1 trial, an experimental drug or treatment is tested in a small group of people (20–80) for the first time. The purpose of this type of trial is to evaluate the treatment’s safety and identify possible side effects.
  • Phase 2 trials: In phase 2 trials, the experimental drug or treatment is given to a larger group of people (100–300) to determine its effectiveness and to further evaluate its safety.
  • Phase 3 trials: In phase 3 trials, the experimental drug or treatment is given to large groups of people (1,000–3,000) to confirm its effectiveness, monitor side effects and compare it with standard or similar treatments.
  • Phase 4 trials: After a drug is licensed and approved by the FDA, researchers track its safety and learn more about its risks, benefits and optimal use in a phase 4 trial.

A placebo is a pill or liquid that looks like the drug being tested in a clinical trial, but has no therapeutic effect. In clinical trials, experimental drugs are often compared to placebos to assess the effectiveness of the experimental drug.

All clinical trials are based on a set of rules called a protocol. A protocol describes who may participate in a clinical trial, the schedule of tests, procedures, medications, dosages and the length of the study.

Principal Investigator
A principal investigator is a doctor or researcher who leads the clinical research team and who, along with the other members of the research team, regularly monitors study participants’ health to determine the study’s safety and effectiveness.

Randomized Study
In a randomized study or trial, participants are assigned by chance to separate groups that compare the different treatments. Neither the researchers nor the participants can choose which group they are in.

Side Effects
Side effects, or adverse reactions, are any undesired effects of a drug or treatment. These may include minor conditions such as a headache, nausea, hair loss or skin irritation, or major events such as liver or kidney damage or other physical problems.