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NIH MedlinePlus the Magazine, Trusted Health Information from the National Institutes of Health

Clinical Trials

What Are Clinical Trials?

two lab workers looking at a test tube

Clinical trials are scientific studies that try to find better ways to prevent, screen for, diagnose, or treat disease. They may also compare a new treatment to one that is already available. There are two basic kinds: observational and interventional.

  • In observational studies, researchers follow people over time, but do not direct them to take certain treatments or to behave in particular ways. A famous example is the Framingham Heart Study. Since 1948, researchers have followed four generations of family members in Framingham, Mass., to see what affects their hearts. Some of its key findings, now widely accepted, are that high blood pressure, high-fat diets, and smoking are not good for the heart.
  • In interventional studies, researchers assign individuals to particular treatments or other interventions and then assess the effect at a predetermined time. For example, researchers may test different drugs to control high blood pressure. Clinical trials in nearly every area of medical research are being conducted all the time.

The Different Phases of Clinical Trials

Clinical trials related to drugs are classified into four phases. The trials at each phase have a different purpose and help scientists answer different questions:

A Phase I trial tests an experimental treatment on a small group of people, to judge its safety and side effects, and to find the correct drug dosage. Sometimes, the study is in healthy people. If the trial is in people with a disease or condition, they receive the experimental treatment, not a placebo (a non-active substance).

A Phase II trial uses more people than a Phase I to find out if the experimental treatment is effective and safe. This phase can last several years.

A Phase III trial is usually a large study with many participants. This phase compares the experimental drug or procedure to a placebo or standard treatment, to make sure it is safe and works well. Some side effects that didn't show up in Phase II may show up in a Phase III trial because many more people are tested. The U.S. Food and Drug Administration will consider the results of clinical trials when determining whether to approve a new drug.

A Phase IV trial takes place after the U.S. Food and Drug Administration approves use of a drug. A drug's effectiveness and safety are monitored in large, diverse populations. Sometimes, the side effects of a drug may not become clear until more people have taken it over a longer period of time.

What Is "Informed Consent"?

Before you decide whether or not to participate in a clinical trial, you need to learn the key facts about it. To help you decide whether or not to participate, the doctors and nurses involved in the trial explain the details of the study. They also give you a document that includes details about the study, such as its purpose, duration, required procedures, risks, potential benefits, and key contacts. You can then decide whether or not to participate based on the information you've received. If you agree to participate, you give your informed consent. Informed consent is not a contract. You may withdraw from the trial at any time.

Fall 2010 Issue: Volume 5 Number 3 Page 5