What are clinical trials?

Clinical trials are scientific studies conducted to find better ways to prevent, screen for, diagnose, or treat disease. These clinical trials may also show which medical approaches work best for certain illnesses or groups of people. Clinical trials produce high-quality data for healthcare decision making.

The purpose of clinical trials is to answer scientific questions. Therefore, these studies follow strict, scientific standards which protect patients and help produce reliable clinical trial results. Clinical trials are one of the final stages of a long and careful research and development process. The process often begins in a laboratory, where scientists first develop and test new ideas.

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:

Phase I trials test an experimental drug, vaccine or device in a small group of people to evaluate safety, possible side effects, and to determine how the drug should be used or delivered.

Phase II trials involve more people than Phase I and they are designed to assess the safety and efficacy of an experimental treatment. This phase can last several years.

Phase III trials are usually large studies with many participants. This phase compares the experimental drug or vaccine to a placebo or standard treatment, to evaluate safety and efficacy. Some side effects that were not identified in Phase II may be identified in a Phase III trial because many more people are evaluated. The regulatory health authority, such as the U.S. Food and Drug Administration, will consider the results of clinical trials up to and including Phase III trials when determining whether to approve a new drug or vaccine.

Phase IV trials take place after a regulatory health authority, such as the U.S. Food and Drug Administration, has approved the use of the drug or vaccine. A drug's effectiveness and safety are monitored in large, diverse populations.

Every clinical trial has a protocol or study plan that describes what will be done during the clinical trial, how the clinical trial will be conducted, and why each part of the clinical trial is necessary. The protocol or study plan also includes guidelines called eligibility criteria for who can and cannot participate in the clinical trial.

Common eligibility criteria may include:

  • Having a certain type or stage of disease
  • Having received (or not having received) a certain kind of therapy in the past
  • Being in a certain age group
  • Medical history
  • Current health status

Criteria such as these help to reduce the medical differences among people taking part in the clinical trial so that researchers can be more certain that the results are due to the treatment being tested and not to another reason.

In addition, because some people who want to take part in a clinical trial have health problems outside of the disease being studied which could be affected by the treatment being evaluated, anyone interested in joining a clinical trial will receive medical tests to confirm their eligibility.