Clinicaltrials How Trials Work

Most clinical trials follow a well-established system. This system is designed to ensure both the well-being of the volunteer and the scientific validity of the study. Clinical trial design is standardized, and human testing occurs only after extensive laboratory study (and sometimes, animal tests) have been completed and evaluated.

Human clinical trial research is broken down into four separate phases:

  • Phase I: Phase I trials identify the safety and/or side effects of the experimental treatment. The study group for Phase I trials is generally small (usually no more than 100 participants).
  • Phase II: Phase II clinical trials continue the work done in Phase I. During this phase, researchers recruit a larger group of participants to determine if the experimental treatment is effective and to further evaluate its safety.
  • Phase III: When Phase III clinical trials start, the need for clinical trial recruiting becomes essential, and thousands of participants may take part in the study. Usually, one group receives the experimental treatment and another group receives either a placebo, or the standard treatment for the condition. These two outcomes are compared for safety and effectiveness. In some cases, this phase can take several years.
  • Phase IV: Phase IV is the last phase of clinical trial design, and the only phase to take place after the Food and Drug Administration (FDA) approves the treatment. During Phase IV, the long-term effectiveness and/or additional uses of the drug may be studied. By necessity, this stage usually lasts several years.

Placebos and Clinical Trials

One purpose of clinical trials to compare experimental treatments with established treatments. Occasionally, volunteers may be given a placebo (medication without the active ingredient of the drug being studied) rather than the experimental drug.

In other cases in which a control group is needed, participants are given the best current treatment instead of a placebo in order to measure the effects of the new drug against what’s currently on the market. This is often done in trials in which the participants have life-threatening illnesses, such as cancer.

The Importance of Randomization

Randomization is an essential part of clinical trial design. In order to ensure that the trials are as fair as possible, volunteers who meet a trial’s criteria are chosen at random for membership in the experimental group, new treatment group or control group. Generally, your physician and the researchers administering the study have no knowledge of your group placement during the trial.

The Clinical Trials Recruitment Process

After you express your interest in a clinical trial, you’ll likely be contacted by phone, email or mail. You will be asked for specific information about your medical history and condition as part of a preliminary screening process to determine if your circumstances meet the trial’s needs. If you fit the profile needed for the study, your name will be added to the list of possible participants. You’ll be notified if you’ve been chosen to participate and told when to meet with the medical personnel running the trial for your first visit.


National Cancer Institute staff. (n.d.) What is randomization? Retrieved January 15, 2010, from the National Cancer Institute Web site:

National Institutes of Health staff. (2008). Glossary of clinical trials terms. Retrieved January 15, 2019, from the Web site:

National Institutes of Health staff. (2008). Clinical trial phases. Retrieved January 15, 2010, from the NIH Web site: