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Clinical trial

Clinical trial

A form of scientific investigation using human volunteers receiving a medical intervention set out by a research plan or protocol.

A clinical trial is a form of scientific investigation using human volunteers receiving an intervention set out by a research plan or protocol. Clinical trials are used to compare the effectiveness of new, or existing, medical treatments or products to standard treatments and/or products, and to a placebo group not receiving an active intervention, or to a group not receiving any intervention. They may try and compare interventions that are already available, or investigate a new intervention which is not known to be helpful, harmful, or the same as alternative interventions; which includes not intervention taking place.

Why clinical trials are performed

Clinical studies are performed for several reasons including:

  • Investigating how one or more interventions, such as drugs, medical devices, surgery procedures, or radiation based therapies, can be used to treat a disease, condition, or syndrome.
  • Discovering ways to prevent recurrences or development of diseases and conditions through medicines, vaccines, lifestyle changes, and other approaches
  • Evaluating how one or more interventions can be used to diagnose or identify a specific disease or condition
  • Examining ways to identify risks associated with a disease or condition
  • Exploring methods and measuring how they can improve quality of life for patients living with chronic illness or are experiencing medical related discomfort
Early Phase 1 (formerly called Phase 0)

Early Phase 1 clinical trials typically involve very limited human exposure to a drug, therapeutic intervention, or medical device and do not have a therapeutic or diagnostic goal. Examples of Early Phase 1 clinical trials include micro-dosing studies and screening studies.

Phase 1

Phase 1 clinical trials focus on establishing the level of safety of an intervention and are typically done with a small amount of healthy human volunteers. The goal of phase one clinical trials is to determine if the invention being studied produces any adverse events, and determine how the intervention is handled by the human body such as how a drug is metabolized and excreted.

Phase 2

Phase 2 clinical trials focus on collecting preliminary data on how an intervention works for people experiencing a particular medical condition. They seek to determine the effectiveness of the intervention in comparison to other interventions, placebo groups, and no treatment groups. Any short-term adverse events produced by the intervention being studied are studied during Phase 2 clinical trials.

Phase 3

Phase 3 clinical trials are performed to describe and gather information on an interventions safety and effectiveness in different populations and intervention exposure levels previously studied in phase 2 trials in a specific population. They typically contain more participants than Phase 1 or Phase 2 clinical trials because the invention has been deemed safe enough to risk exposing more people and further experimenting with levels of exposure to an intervention. Phase 3 clinical trials are the last set of clinical trials before an intervention can be approved by the Food and Drug Administration (FDA) approves an intervention for marketing.

Phase 4

Phase 4 clinical trials are performed after an intervention has been approved by the FDA for marketing purposes. They are done to perform commitment and postmarket requirement studies required, or are agreed to, by the interventions sponsor. Phase 4 clinical trials are also used to collect more information about an interventions safety, efficacy, and optimal use.

Common questions about clinical trials

Individuals participating, sponsoring, or desiring to learn more about clinical trials in general, or about specific clinical trial, commonly ask similar questions about clinical trials to help them participate in discussions and better understand clinical trials. Commonly asked questions about clinical trials include:

  • What is being studied?
  • Why do the investigators believe the intervention is worth studying?
  • Why might the intervention being studied not be effective or safe?
  • Have similar clinical trials been done in the past?
  • What are the possible interventions that patients may receive during the clinical trial?
  • What will patients have to do before, during, and after the clinical trial?
  • Who will know what patients receive which intervention? Will the investigator know? Will the research team know? Will the patient know?
  • What are the potential risks, side effects, and benefits of the intervention used in the clinical trial when compared to established options?
  • What tests and procedures are involved in the clinical trial?
  • How often do the participants have to visit the location the clinical trial in being held?
  • How long does the clinical trial last?
  • Will hospitalization or extended time in one location be required?
  • Who is paying for the clinical trial to take place?
  • Are the patients being compensated for their participation in the clinical trial?
  • Will patients be reimbursed for expenses they may occur during the clinical trial?
  • What form of long-term follow-ups are required as part of the clinical trial?
  • If patients experience positive outcomes during the clinical trial, are they able to continue receiving the intervention when the clinical trial ends?
  • Who will the results of the clinical trial be available to?
  • Who is responsible for each patients medical care during their participation in the clinical trial?
  • What are the options for someone who is harmed as a result of participating in the clinical trial?
Companies in the clinical trial industry


Further Resources


Clinical Trials: MedlinePlus

Medicine Plus


Deciding to Participate in Clinical Trials


December 27, 2016

NIH Clinical Research Trials and You

National Institutes of Health


Observational and interventional study design types; an overview


Regulations to Protect Research Volunteers

Office for Human Research Protections (OHRP)


January 28, 2020


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