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Clinical Trials Frequently Asked Questions

Clinical Trial Glossary
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Frequently asked questions: Study participants

Clinical trials are studies that evaluate the safety and effectiveness of drugs, medical devices or other medical treatments when tested on human beings. Clinical trials follow a defined study plan known as a protocol. These trials are often referred to as clinical studies, medical research studies or just studies.

Clinical trials are conducted by medical personnel, such as doctors and nurses. Doctors who conduct clinical trials are also called investigators. The clinical trial team, or site staff, may also include social workers and other specialized health care professionals.

The investigators will: 

  • Check your general health at the beginning of the trial
  • Give you specific instructions for participating in the trial
  • Closely monitor your safety and check your progress throughout the trial
  • Stay in touch with you after the trial is completed

What happens in a clinical trial depends on the trial itself. However, you will most likely need to visit the trial clinic at some point to receive instructions and have tests or evaluations performed. In some clinical trials, there may be more tests and trial-related visits than you would normally have for a particular illness or medical condition. These additional medical tests and clinic visits help site staff monitor your general health and safety throughout the trial carefully.

  • Treatment trials test investigational treatments, new combinations of drugs, or new approaches to surgery or radiation therapy.
  • Prevention trials look for better ways to prevent a given disease from occurring in people who have never had the disease or prevent the disease from returning. These approaches may include medicines, vitamins, vaccines, minerals or lifestyle changes.
  • Diagnostic trials are conducted to find better medical tests or procedures for diagnosing a particular disease or medical condition.
  • Screening trials test the best way to detect certain diseases or health conditions.
  • Quality of life trials (or supportive-care trials) explore ways to improve comfort and the quality of life for individuals with chronic illnesses.

For participants with specific illnesses or medical conditions, there are two major types of clinical trials:

  • Interventional: During this type of trial, you are given a specific treatment defined by the protocol, and the results are measured by investigators.The investigators in interventional studies take an active role by administering a drug or a medical device and measuring its effects.
  • Observational: In this type of trial, you are not given any specific treatment; but you are observed by the site staff and any changes in your medical condition are closely monitored. The results of this trial are measured by investigators. In observational studies, the investigators take a passive role, only observing associations between volunteers' current treatments and their health status. No treatments are administered related to the trial.

Clinical trials are sponsored or funded by a variety of organizations and individuals, such as physicians, medical institutions, foundations, volunteer groups, pharmaceutical companies and government agencies.

Trials can take place in a variety of locations, such as hospitals, universities, doctors' offices or community clinics.

Ideas for clinical trials usually come from researchers who may work at pharmaceutical companies, universities, hospitals or in government programs. Researchers test new therapies or procedures in the laboratory and in other studies. The investigational treatments with the most promising laboratory results are moved into clinical trials. During a clinical trial, researchers gather more information about an investigational treatment, its risks and how well it may or may not work.

Clinical trials that are well designed and well executed are the best way for you to:

  • Play an active role in your own health care
  • Gain access to new medical treatments before they become widely available
  • Obtain expert medical care at leading healthcare facilities during the trial
  • Help others by contributing to medical research

However, every clinical trial has some risks, including:

  • There may be unpleasant, serious or even life-threatening side effects to the investigational treatment
  • The investigational treatment may not be effective for you
  • The protocol may require more of your time and attention then would a non-protocol treatment, including trips to the clinic, more medical treatments, hospital stays or complex dosage requirements

Any potential risks related to a clinical trial will be fully explained to you before you enroll in the study.

Clinical trials are conducted in phases. The trials at each phase have a different purpose and help scientists answer different questions:

  • Phase I Trials - In Phase I trials, researchers test an investigational drug or treatment in a small group of people (20-80) to evaluate its safety, determine a safe dosage range, and identify any side effects. Many times Phase I trials are conducted using only healthy volunteers. Phase I trials are the first time an investigational treatment is tested on humans; however, substantive research with and testing of the investigational medication has already occurred, with favorable results, to get to this stage of the process.
  • Phase II Trials - In Phase II trials, the investigational drug or treatment is given to a larger group of people (100-300) with the targeted medical condition to see if it is effective and to further evaluate its safety.
  • Phase III Trials - In Phase III trials, the investigational drug or treatment is given to large groups of people (1,000-3,000) with the targeted medical condition to confirm its effectiveness, monitor side effects, compare it to commonly used treatments, and collect information that will allow the investigational treatment to be used safely.
  • Phase IV Trials - In Phase IV trials, additional information, including the drug's risks, benefits, and optimal use, is collected.

Most studies do not require you to stay overnight in the study clinic. Some trials involve a weekend or two in a clinic, while other trials require longer periods at the clinic of up to several weeks (more common in Phase I studies with healthy volunteers). Post-study follow-up visits, called "outpatient visits," for blood samples are sometimes necessary.

Also, many participants qualify for multiple trials. However, you normally cannot participate in more than one trial at a time, and there is a minimum waiting period between trials. This waiting period is known as the "wash-out period" and normally varies between one and four months, depending on the medicine that was tested. Also, if you are a blood donor, you will need to meet a minimum wash-out period for blood donation, which will vary from trial to trial.

When describing drugs, medications and medical devices that are not approved by the FDA or other governing agency for public use, researchers often use the word "investigational." Some researchers may also refer to these treatments as "experimental."

A protocol is a study plan for a clinical trial that is carefully designed to safeguard your health and answer specific medical research questions. A protocol details:

  • Who may participate in the clinical trial
  • The schedule of tests, procedures, medications and dosages
  • The length of the clinical trial
  • How often and when you are to be seen by the site staff so they can monitor your health and determine the safety and effectiveness of your treatment

A placebo, sometimes called a sugar pill, is a substance (pill, liquid or powder) that has no active medication. In clinical trials, investigational treatments are often compared with placebos. In some trials, people in the control group receive a placebo instead of an active drug or investigational treatment.

A control is the standard against which the effects of the clinical trial treatment are measured. A control group might receive the existing standard treatment for an illness, or they may receive no treatment at all.

In many clinical trials, one group of patients will be given an investigational drug while the control group is given either a standard treatment for the illness or a placebo, a substance that looks like the investigational drug but contains no active medication. It is very important that clinical trial participants and site staff do not know which treatment the participant receives. This ensures that any existing opinions about the investigational drug and/or placebo do not affect final trial results.

Side effects are any undesired actions or effects of an investigational drug or treatment. Negative, or adverse, effects will vary depending on the medication being studied. One of the goals of clinical trials is to evaluate investigational treatments for both immediate and long-term side effects. The site staff will discuss this with you in more detail before you sign up for the clinical trial.

The investigator, or clinical trial doctor, will provide you with detailed information about the trial to allow you to decide whether or not to participate. The process of understanding the risks and benefits of a trial is known as informed consent. The doctor will continue to keep you informed throughout the trial and will provide you up-to-date information.

The investigator will provide an informed consent document that includes details about the study, such as:

  • Purpose of the research
  • Trial duration/how long it will last
  • Details about the investigational treatment
  • Medical tests and procedures required by the trial
  • Key trial contacts
  • Risks of participation
  • Potential benefits of participation

You should know as much as you can about the clinical trial and feel comfortable asking your doctor any questions you may have related to the trial. Once you are comfortable with your understanding of what is involved, you will have the opportunity to decide whether or not to sign the informed consent document, stating that you are willing to participate. However, keep in mind that the informed consent document is not a contract, and you may withdraw from the clinical trial at any time and for any reason.

Before joining a clinical trial, you must meet certain criteria for participation. Each clinical trial has a unique set of guidelines about who can participate. The guidelines are used to:

  • Identify appropriate participants
  • Protect participants' safety
  • Help ensure that investigators will be able to answer their research questions

Guidelines for participation can be based on:

  • Age
  • Gender
  • For some studies, the type and stage of your disease
  • Previous or current treatments and medications
  • Other existing medical conditions

No. Some clinical trials focus only on how the medicine acts in the bodies of healthy volunteers – how the compound is absorbed, distributed, metabolized and excreted (see What are the phases of clinical trials?). Other clinical trials, such as the testing of vaccines, focus on how to prevent illness.

The same ethical and legal codes that apply to doctors also apply to clinical trials. In addition, most clinical research is regulated by governments, with built-in safeguards to protect the participants.

Every clinical trial in the United States must be approved and monitored by an Institutional Review Board (IRB) to make sure the risks are as low as possible and are worth any potential benefits. An IRB is an independent committee of physicians, statisticians, community advocates and others that ensures a clinical trial is ethical and the rights of participants are protected. Many countries have similar review boards and regulations.

Yes. You may leave a clinical trial at any time and for any reason. When withdrawing from the trial, you are encouraged to let the site staff know, and also give your reasons for leaving the clinical trial.

To prepare for the initial meeting with the site staff, you should:

  • Write down any possible questions you might have
  • Ask a friend or relative to come along for support and to hear the responses to your questions
  • Bring a tape recorder to record the discussion and to replay later

You can also find a list of questions for the study doctor on the Questions to Ask Your Doctor page.

The following questions might be helpful for you to discuss with the investigator. You can find additional questions on the Questions for Your Doctor page.

Yes. Most clinical trials provide short-term treatments related to a designated illness or medical condition but do not provide extended or complete primary health care. In addition, by having your primary health care provider work with the site staff, you can help ensure that other medications or medical treatments will not conflict with the protocol.

Yes. The health and well-being of every clinical trial participant is of the highest priority. Qualified medical staff will be available at all times.


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Information included in the Glossary is from Clinicaltrials.gov, which uses content from AIDSinfo, CenterWatch, Inc., ECRI, Eli Lilly and Company, MediStudy.com, Inc. and National Cancer Institute.