Knowledge Center

Clinical Trials Knowledge Center – Articles, FAQ & Glossary

Clinical Trials Frequently Asked Questions

Clinical Trial Glossary

Glossary results: P

Review of a clinical trial by experts chosen by the study sponsor. These experts review the trials for scientific merit, participant safety and ethical considerations.

The processes (in a living organism) of absorption, distribution, metabolism and excretion of a drug or vaccine.

Initial studies to determine the metabolism and pharmacologic actions of drugs in humans, the side effects associated with increasing doses and to gain early evidence of effectiveness. May include healthy participants and/or patients.

Controlled clinical studies conducted to evaluate the effectiveness of the drug for a particular indication or indications in patients with the disease or condition under study and to determine the common short-term side effects and risks.

Expanded controlled and uncontrolled trials after preliminary evidence suggesting effectiveness of the drug has been obtained. Intended to gather additional information to evaluate the overall benefit-risk relationship of the drug and provide an adequate basis for physician labeling.

Post-marketing studies to delineate additional information including the drug's risks, benefits and optimal use.

A placebo is an inactive pill, liquid or powder that has no treatment value. In clinical trials, experimental treatments are often compared with placebos to assess the treatment's effectiveness.

A method of investigation of drugs in which an inactive substance (the placebo) is given to one group of participants, while the drug being tested is given to another group. The results obtained in the two groups are then compared to see if the investigational treatment is more effective in treating the condition.

A physical or emotional change, occurring after a substance is taken or administered, that is not the result of any special property of the substance. The change may be beneficial, reflecting the expectations of the participant and, often, the expectations of the person giving the substance.

Post-approval studies test a marketed drug in new age groups or patient types. Some studies focus on previously unknown side effects or related risk factors. As with all stages of drug development testing, the purpose is to ensure the safety and effectiveness of marketed drugs.

Refers to the testing of experimental drugs in the test tube or in animals-the testing that occurs before trials in humans may be carried out.

Refers to trials to find better ways to prevent disease in people who have never had the disease or to prevent a disease from returning. These approaches may include medicines, vaccines, vitamins, minerals or lifestyle changes.

A principal investigator, usually a scientist or physician, is the person with primary responsibility for initiating and conducting a clinical trial. This person is responsible to see that a trial is approved by a human subject review board and must conform to the regulations of the appropriate national health authorities, in order to be registered.

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. While in a clinical trial, participants following a protocol are seen regularly by the research staff to monitor their health and to determine the safety and effectiveness of their treatment. provides links to other Internet sites for the user's convenience. is not responsible for the availability or content of these external sites, nor does it endorse, warrant or guarantee the products, services or information described or offered at these sites.

Information included in the Glossary is from, which uses content from AIDSinfo, CenterWatch, Inc., ECRI, Eli Lilly and Company,, Inc. and National Cancer Institute.