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Clinical Trials

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Also known as clinical research

Clinical trials are medical studies that involve people like you. They help find new ways to prevent, detect, or treat diseases that are safe and effective. The National Heart, Lung, and Blood Institute (NHLBI) leads and supports many studies aimed at preventing, diagnosing, and treating heart, lung, blood, and sleep disorders.

Clinical trials are an important part of the research spectrum. The idea for a clinical trial often starts in the lab. After researchers test new treatments or procedures in the lab and in animals, the most promising treatments are moved into clinical trials. As studies about new treatments move through a series of steps called phases, researchers learn more information about the treatment, its risks, and its effectiveness.

Each clinical trial has criteria describing who can join. Children as well as adults, healthy volunteers and patients, and people of a diverse range of ethnic and racial backgrounds can and are encouraged to participate in clinical trials.

Clinical trials follow a plan, called a protocol, that describes what you will be doing and what you can expect from the research team. It is important to understand the risks and benefits of participation before joining. You also have rights and protections as a participant in clinical trials.


Purpose of clinical trials

An Ebola vaccine is in clinical trials at Walter Reed Army Institute for Research.

Clinical trials have different purposes. What that purpose is helps define the type of trial it is.

  • Behavioral trials evaluate or compare ways to promote behavioral changes designed to improve health.
  • Diagnostic trials study or compare tests or procedures for diagnosing a particular disease or condition.
  • Prevention trials look for better ways to prevent a disease in people who have never had the disease or to prevent the disease from returning. Approaches may include medicines, vaccines, or lifestyle changes.
  • Quality of life trials, or supportive care trials, explore and measure ways to improve the comfort and quality of life for people with conditions or illnesses.
  • Screening trials test new ways for detecting diseases or health conditions.
  • Treatment trials test new treatments, new combinations of medicines, or new approaches to surgery or radiation therapy.

Clinical trial phases

Researchers conduct clinical trials in a series of steps called phases. Each phase has a different purpose and helps researchers answer different questions.

  • Phase I trials. Researchers test a medicine or other treatment in a small group of people for the first time. The purpose is to learn about the best dose, if it is a medicine, as well as its safety and side effects.
  • Phase II trials. Researchers study the new medicine or treatment in a larger group of people to determine its effectiveness and to further study its safety.
  • Phase III trials. Researchers give the new medicine or treatment to an even larger group of participants to confirm its effectiveness, monitor side effects, compare it with standard or similar treatments or a placebo, and collect information that will allow the new medicine or treatment to be used safely.
  • Phase IV trials. After the U.S. Food and Drug Administration (FDA) approves a medicine and it is made available to the public, researchers track its safety in the general population, seeking more information about a medicine or treatment’s benefits and optimal use.

Look for

  • Read What to Expect to learn about the teams running and supporting a clinical trial, the plan that they follow, and common terms in clinical trial design.

What to Expect

As a participant in a clinical trial, you may work with a healthcare team, and you may need to go to a hospital or other location. Everything that happens throughout your experience follows a plan called a clinical trial protocol. Governing bodies called Institutional Review Boards (IRBs) approve protocols and are responsible for ensuring your safety. The research team will also operate by other national and international standards that protect you and help produce reliable study results. The NHLBI is one of many types of organizations that support clinical trials. Before you join a clinical trial, you will be told all about the study, what procedures you will be undergoing, how much time you will be spending on aspects of the study, and any other information you need to know. Once your questions have been answered and you are comfortable, you will be asked to give your consent to participate.

Clinical trial experience

During a clinical trial, you may see doctors, nurses, social workers, and other healthcare providers who will monitor your health closely. You may have more tests and medical exams than you would if you were not taking part in a clinical trial. You may also be asked to do other tasks, such as keeping a log about your health or filling out forms about how you feel.

You may need to travel or stay in a hospital to take part in clinical trials. Many clinical trials take place in medical centers and doctors’ offices around the country. If you decide that a trial is not for you, it is important to remember that you can withdraw at any time. Whether you participate or not will not affect your regular medical care.

Clinical trial protocols

Clinical trials follow a plan known as a protocol. The protocol is carefully designed to balance the potential benefits of a trial with the risks to participants. It also answers specific research questions. A protocol describes the following:

  • Goals of the study
  • Eligibility requirements
  • Protections against risks to participants
  • Details about tests, procedures, and treatments
  • Expected duration, or how long the study will last
  • Information to be gathered

A clinical trial team is led by a principal investigator (PI). Members of the research team regularly monitor the participants’ health to determine the study’s safety and effectiveness.

Clinical trial designs

There are different types of clinical trials and different trial designs. However, many clinical trials include standard design elements.

Randomization is the process by which participants are randomly assigned a treatment instead of being selected for one or the others. This is done to avoid bias when making assignments. The effects of each treatment are compared at specific points during a trial. If one treatment is found superior, the study is stopped so that all the volunteers receive the more beneficial treatment.

Blinded or masked studies are designed to prevent members of the research team and study participants from influencing the results. Blinding allows the collection of scientifically accurate data.

  • In single-blind (single-masked) studies, you are not told what is being given, but the research team knows.
  • In a double-blind study, neither you nor the research team are told what you are given; only the pharmacist knows. Members of the research team are not told which participants are receiving which treatment, in order to reduce bias. If medically necessary, however, it is always possible to find out which treatment you are receiving.

When the study is finished

After a clinical trial is completed, the researchers carefully examine information collected during the study before making decisions about the meaning of the findings and about the need for further testing. After a phase I or II trial, the researchers decide whether to move on to the next phase or to stop testing the treatment or procedure because it was unsafe or not effective. When a phase III trial is completed, the researchers examine the information and decide whether the results have medical importance.

Results from clinical trials are often published in scientific journals in articles that have gone through peer review. Results that are particularly important may be featured in the news, and discussed at scientific meetings and by patient advocacy groups. Once a new approach has been proven safe and effective in a clinical trial, it may become a new standard of medical practice. In many cases, if you participated in a blinded or masked study, you will get information about the treatment you received

Ask the research team members if the study results have been or will be published. Published study results are also available by searching for the study's official name or Protocol ID number in the National Library of Medicine's PubMed® database.

Who Can Participate

Many different types of people take part in clinical trials. Some studies include healthy volunteers, while other studies include patient volunteers. Some studies include both healthy and patient volunteers. In addition, the NHLBI is committed to supporting clinical trials with diverse participants. This includes studies specific to children’s needs. Eligibility criteria determine who can participate in a clinical trial.

How much of your time is needed, discomfort you may feel, or risk involved depends on the clinical trial. While some studies require minimal amounts of time and effort, other studies may require a major commitment of your time and effort and may involve some discomfort. The clinical trial may also carry some risk. The informed consent process for volunteers includes a detailed discussion of what you will be asked to do as part of the study and any possible risks.

Healthy volunteers

Clinical trials with healthy volunteers are designed to develop new knowledge, not to provide direct benefit to those taking part. Researchers take measurements and make observations. Researchers may use the data to compare patient volunteers and healthy volunteers.

Patient volunteers

Research with patient volunteers also helps develop new knowledge. Depending on the stage of knowledge about the disease or condition, these procedures may or may not benefit the patient volunteer.

Patients may volunteer for studies similar to those in which healthy volunteers take part.

Diverse volunteers

In the past, clinical trial volunteers often were white men. Researchers assumed that study results were valid for other populations as well. Today, researchers realize that women and people from different racial and ethnic groups sometimes respond differently than white men to the same medical approach. As a result, the NIH and the NHLBI are committed to supporting clinical trials that include both men and women as well as racially and ethnically diverse populations.


Children need clinical trials that focus on them, as medical treatments and approaches often differ for children. For example, children may need lower doses of certain medicines or smaller medical devices. A child’s stage of development also can affect how safe a treatment is or how well it works.

Eligibility criteria

A clinical trial's protocol describes who is eligible to take part in the research. Each study must include only people who meet the requirements for that study. These are the study's eligibility criteria.

Eligibility criteria are different for each trial. They include whether you are a healthy or patient volunteer. They also include factors such as your age and sex, the type and stage of disease, and whether you have had certain treatments or have other health problems.

The criteria ensure that new approaches are tested on similar groups of people. This makes it clear to whom a clinical trial's results apply. Eligibility criteria are not used to reject people personally. Instead, the criteria are used to identify appropriate participants and keep them safe, and to help ensure that researchers can find the new information they need.

Benefits and Risks

Clinical trials offer hope for many people, while giving researchers a chance to find treatments that could benefit patients in the future. Healthy volunteers say they take part to help others and contribute to moving science forward. People with an illness or disease may take part to help others, but also to have a chance to receive the newest treatment and get added care and attention from the clinical trial staff. Clinical trials may involve risk, as can routine medical care and the activities of daily living. When weighing the risks of clinical trials, consider the possible harms that could result from taking part in the study, the level of harm, and the chance of any harm occurring.

Possible benefits

  • Well-designed and well-performed clinical trials provide benefits to you, while allowing you to help others by contributing to knowledge about new treatments or procedures.
  • You gain access to new research treatments before they are widely available.
  • You receive regular and careful medical attention from a research team that includes doctors and other healthcare professionals.

Possible risks

Clinical trials do come with some risks.

  • Most clinical trials pose the risk of minor discomfort, which often lasts only a short time. However, some study participants experience complications that require medical attention. Rarely, participants have experienced serious or life-threatening complications resulting from their participation in trials of experimental treatments.
  • The study may require more time and attention than standard treatment would, including visits to the study site, more blood tests, more procedures, hospital stays, or complex dosage schedules.

In clinical trials that compare a new product or therapy with another that already exists, researchers try to determine if the new one is as good, or better than, the existing one. In some studies, you may receive a placebo. Comparing a new product with a placebo can be the fastest and most reliable way to show the new product's effectiveness. However, placebos are not used if you would be put at risk—particularly in the study of treatments for serious illnesses—by not having effective therapy. You will be told if placebos are used in the study before entering a trial.

You can learn about the risks and benefits of any clinical trial and how your rights are protected before you agree to take part in the trial. A member of the research team will explain the study and answer any questions about the study. A member of the research team will also ask you to consider and sign an informed consent document, which will describe in detail the specific risks associated with a research protocol. Before deciding to participate, carefully consider risks and possible benefits. You can also talk with your doctor about specific clinical trials you are interested in.

Protecting Your Safety

Protecting the safety of people who take part in clinical trials is a high priority for the NHLBI and its researchers. You also have rights to help protect your safety.

Scientific oversight

Each study has scientific oversight, including the following:

  • Institutional Review Board (IRB). An IRB approves and monitors most, but not all, clinical trials in the United States to ensure that the risks are minimal when compared with potential benefits. An IRB is an independent committee that consists of physicians, statisticians, and members of the community who ensure that clinical trials are ethical and that the rights of participants are protected. You should ask the research coordinator if an IRB reviewed the research in which you are considering participating.
  • Office for Human Research Protections. The U.S. Department of Health and Human Services' (HHS') Office for Human Research Protections (OHRP)external link oversees all research done or supported by HHS. The OHRP helps protect the rights, welfare, and well-being of healthy volunteers and patient participants. The OHRP provides guidance and oversight to the IRBs, develops educational programs and materials, and offers advice on research-related issues.
  • Data and Safety Monitoring Board. Many clinical trials supported or conducted by NIH are required to have a Data and Safety Monitoring Board (DSMB). This board consists of a group of research and study topic experts. A DSMB's role is to review data from a clinical trial for safety problems or differences in results among different groups. The DSMB also reviews research results from other relevant studies. Scientific oversight informs decisions about a study while it is underway. For example, researchers stop some studies early if benefits from a strategy or treatment are obvious to make wider access to the new strategy available sooner. Researchers also may stop a study, or part of a study, early if the intervention or treatment is having harmful effects.
  • FDA. In the United States, the FDA provides oversight for clinical trials that are testing new medicines or medical devices. The FDA reviews applications for new medicines and devices before studies begin with human volunteers. They check to make sure that the proposed studies have followed proper informed consent and protection procedures. The FDA also provides oversight and guidance at various stages throughout the studies. For example, before Phase III trials begin, the FDA provides input on how these studies should be done.

Patient rights

As a participant in a clinical trial, you have rights that help protect your safety. These rights include:

  • Informed consent. Informed consent is the process of learning the key facts about a clinical trial before deciding whether to participate. The process of providing information to participants continues throughout the study. To help you decide whether to take part, members of the research team explain the study. The research team provides an informed consent document, which includes such details about the study as its purpose, duration, required procedures, and who to contact for various purposes. The informed consent document also explains risks and potential benefits. If you decide to enroll in the trial, you will need to sign the informed consent document. You can withdraw from the study at any time.
  • Rights and protection for children. Children under the age of 18 get special protection as research participants. Almost always, a parent must give legal consent for his or her child to take part in a clinical trial. Sometimes, both parents must give permission for their child to enroll. Also, children age 7 and older often must agree to take part in clinical trials.


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