A Guide to Understanding Clinical Trials

Updated:Nov 13,2015

Clinical trials are scientific studies that determine if a possible new medical advance can help people and whether it has harmful side effects. The people who volunteer to be studied during these trials are advised beforehand of possible risks. Before a clinical trial can begin for a treatment, medication or device, it must show promising results in laboratory or animal tests.

Below are some common questions about clinical trials, with answers from the National Institutes of Health:


What happens during a clinical trial?

At the beginning of a clinical trial, a team of doctors, nurses, social workers and other health professionals checks the health of the participant and gives specific instructions for using the treatment, medication or device throughout the trial. The team monitors the participant as the trial progresses and stays in touch after the trial is completed. All clinical trials follow a detailed plan called a protocol. 

Are clinical trials safe?

For the most part, yes. However, sometimes clinical trials are for experimental medications intended for very sick people, so in those cases there are generally more risks involved. The patient will always be advised of the risks involved. Every clinical trial in the U.S. must be approved and monitored by an Institutional Review Board to ensure that it is ethical and the rights of study participants are protected. 

Who can participate in a clinical trial?

All clinical trials have guidelines about who can participate. Criteria may include age, gender, the type and stage of a disease, previous treatment history and other medical conditions.

Why participate in a clinical trial?

You can access new research treatments before they are widely available, and your participation also may help others by contributing to medical research.

Do I have to stay in a hospital or leave my home for a clinical trial?

Participants are generally asked to come in for regular office visits at an accessible clinical trial site, which may be a doctor’s office, a medical center or community hospital. Some trials involve occasional overnight hospital stays or require more doctor visits than would be normal for an illness or condition. Many trials involve a physical examination or medical history review after enrollment. A description of what’s expected of the participant should be made available before signing the informed consent document. 

What kind of paperwork or legal agreements do I have to sign?

The directors of a clinical trial explain the risks and potential benefits involved. This information is also provided in an informed consent document, which the participant is required to sign before taking part in the trial. According to the FDA, minimum required elements of an informed consent include the following:

  1. Purpose of research
  2. Foreseeable risks
  3. Potential benefits
  4. Disclosure of alternative procedures
  5. Confidentiality
  6. Course of action if more than minimal risk involved
  7. Who to contact
  8. Participation is voluntary

Can you quit after a clinical trial starts?

Yes. You can leave a clinical trial at any time.

How long do clinical trials typically last?

They vary greatly. Some last just a few days while others take years. 

If I decide to take part in a clinical trial, what should I do to prepare?

You should learn as much about the trial as possible before signing the informed consent document. Helpful questions to ask the trial directors include:

  • What is the purpose of the study?
  • Who is going to be in the study?
  • Why do researchers believe the experimental treatment being tested may be effective? Has it been tested before?
  • What kinds of tests and experimental treatments are involved?
  • How do the possible risks, side effects and benefits in the study compare with my current treatment?
  • How might this trial affect my daily life?
  • How long will the trial last?
  • Will hospitalization be required?
  • Who will pay for the experimental treatment?
  • Will I be reimbursed for other expenses?
  • What type of long-term follow-up care is part of this study?
  • How will I know that the experimental treatment is working? Will results of the trials be provided to me?
  • Who will be in charge of my care?

Where can I find understandable definitions of some of the scientific terms used during a clinical trial?

The National Institutes of Health Clinical Trials Glossary lists more than 80 terms commonly used in clinical trials.  

Where can I get more detailed information about clinical trials?

Helpful informational sources include:


How can I be sure a clinical trial is legitimate?

The trial’s research staff should be willing to answer all of your questions, such as who is sponsoring the trial, whether the trial protocol is compliant with federal regulations, what is the purpose of the trial, what are the possible risks and how long the trial is expected to last.

A reliable starting point is the National Institutes of Health Clinical Trial Registry at http://clinicaltrials.gov. This site lists more than 103,000 trials in more than 170 countries. You can search for a trial using criteria such as condition or disease treated, medication or therapy used, or site country. Each entry includes a trial description, sponsors, purpose, estimated completion date, eligibility criteria and contact information.

The World Health Organization International Clinical Trials Registry Platform provides similar background information about trials across the worldwide.

What are the different types of clinical trials?

There are several kinds of trials, each with a different purpose:

  • Treatment trials test experimental treatments, new combinations of drugs or new approaches to surgery or radiation therapy.
  • Prevention trials look for better ways to prevent disease in people who have never had the disease or to prevent a disease from returning.
  • Diagnostic trials evaluate tests or procedures for diagnosing a disease or condition.
  • Screening trials evaluate methods of detecting a disease or health condition.
  • Quality of Life trials (or Supportive Care trials) evaluate methods of improving the comfort and quality of life of patients with chronic illness.

Are there different phases of clinical trials?

To be approved by the U.S. Food and Drug Administration, an experimental drug or treatment has to pass through several phases of clinical trials. 

Phase I trials test an experimental drug or treatment in a small group of people (20-80) for the first time to evaluate its safety and identify side effects.

After safety has been confirmed, phase II trials test the experimental drug or treatment in a larger group of people (100-300).

Phase III trials test the experimental drug or treatment in a large group of people (1,000-3,000) to confirm its effectiveness. Usually, two successful trials are required before a drug or treatment is approved by the U.S. Food and Drug Administration.

Phase IV trials acquire additional information about an experimental drug or treatment after it has been approved.

Who sponsors clinical trials?

Clinical trials can be sponsored by organizations or individuals such as physicians, medical institutions, foundations, voluntary groups and pharmaceutical companies, in addition to federal agencies such as the National Institutes of Health, the Department of Defense and the Department of Veterans Affairs.

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