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Clinical Studies: Information for the Public and Potential Volunteers

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What is a clinical study?

A clinical study is a research study (sometimes called a trial or protocol) in which people agree to participate after providing informed consent. These are carefully designed studies that aim to uncover better ways to prevent, diagnose, or treat conditions, or to better understand specific conditions or processes.

One of the most common types of clinical studies is a clinical trial, in which researchers work with volunteers to find out how a substance or action, also called an intervention, affects a person. Interventions are, for example, drugs, therapies or procedures, medical devices, or lifestyle changes (behaviors), such as changes in diet or level of physical activity. Clinical trials are often designed to see if the intervention prevents, finds, or improves a disease or condition. 

Clinical trials may compare experimental interventions with existing products or processes, or with a product or process that appears to be identical to the intervention being studied but which has no known effect, such as a dummy pill (called a placebo). Clinical trials can also compare two or more existing approaches to determine which is safer or more effective.

In randomized clinical trial, neither the volunteer nor the researcher know which intervention the volunteer is receiving. This ensures that the results are not impacted by any assumptions or preconceived notions of how the intervention will affect the volunteer.

Before being tested on people during in clinical trials, the drug, device, or other approach being studied has been shown to be safe and helpful in a laboratory (such as in studies of cells or tissues in test tubes) and, sometimes, in animals.

Another type of clinical study is an observational study. Observational studies are key to learning about the incidence, prevalence, and prognosis of a disease or condition. Population-based research studies—which are a type of observational study—are critical for estimating how many people are affected by specific conditions and the burden those conditions have on society.

Like clinical trials, researchers collect information about participants, including how treatments or other factors affect the volunteers; however, in observational studies, researchers do not play a role in assigning  the interventions. Observational studies therefore enable researchers to gain key insights into factors (sometimes called determinants) that may be linked to another condition or health outcome. In addition to medicines and other treatments, these factors may include, for example, behaviors, age, gender, and health conditions.  

Who can participate in a clinical study?

Clinical studies enroll people—volunteers—who are alike in certain ways, depending on the study's purpose. The study's protocol tells who can join the study and spells out the characteristics that volunteers should have. These are called eligibility criteria. They may include age, gender, general health, and other risk factors. Many studies include healthy volunteers—people with no known significant health problems—as well as people with certain conditions or risk factors for a disease or illness.

Eligibility criteria are a key part of medical research. They help produce results we can trust. After those results are known, the information can help researchers find out who will be helped by the approach being studied—if it's shown to work. For example, a new drug might not work for people with one type of risk factor, or it may work better for men than for women. Eligibility criteria also help protect study participants. They help make sure that if a volunteer is likely to be harmed by something in the study, he or she is not exposed to that risk.

Researchers study only individuals who volunteer to participate in their study. A person who is interested in volunteering is informed about the risks and benefits of taking part in the study, including details about the study approach and any tests that may be performed. When a person decides to participate, he or she signs a consent form. This process is called informed consent, and it lasts as long as the person is in the study. In other words, a volunteer can change his or her mind and leave a study at any time.

How can I find studies seeking volunteers?

Most research funded by the National Institutes of Health (NIH)—which includes the NIDCD—is conducted at universities, medical schools, and other institutions nationwide and around the world. This includes research conducted in laboratories as well as clinical research. Similarly, clinical research studies are conducted in sites throughout the United States., a website maintained by the NIH, provides current information about clinical studies for potential volunteers, family members, health providers, and others. You can search the entire database or go directly to a list of NIDCD-supported clinical studies. is another tool to connect volunteers with appropriate studies. This free, confidential registry was developed in part through NIH funding. After you register on the site, you will receive emails about any studies that appear to be relevant, with instructions about how you can get more information about participating.

NIDCD scientists who work at the NIH in Bethesda, Maryland, also conduct clinical research in communication disorders, primarily in hearing and balance. These scientists and their staff are part of the NIDCD Division of Intramural Research. Most of their clinical studies are conducted at the NIH Clinical Center, the world’s largest complex devoted to clinical research. Here are the current NIDCD clinical studies at the NIH.

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