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Types of Clinical Trials

When a new product or approach is being studied, it is not usually known whether it will be helpful, harmful or no different than available alternatives. Investigators try to determine the safety and efficacy of the intervention by measuring certain outcomes in the participants. 

Phase 0 — Also known as “exploratory studies,” these trials test a small dose of a new drug in a few people as researchers explore how the drug may work in humans and ensure that it is safe. Trial participants may or may not have the disease the drug has been developed to treat. 

Phase 1 — In these trials, or “safety studies,” researchers focus on whether an experimental treatment is safe while determining how best to administer the treatment and in what dose. Placebos are not used, and these studies typically include a small number of participants.  

Phase 2 — If an experimental treatment proves safe in phase 1 clinical trials, it may then be tested in phase 2 “efficacy” trials. These studies are designed to assess whether an experimental treatment works. The trial may involve healthy volunteers or people with the targeted condition, and some participants may be randomly assigned to different groups that receive a different dose or treatment protocol.  

Phase 3 — In this “confirmatory” phase, the number of participants is increased and the experimental drug often is tested against a placebo in order to prove that the drug is both effective and better than any existing treatments. 

Phase 4 — Approved drugs are monitored over a period of time in phase 4 studies. Although these treatments are available for doctors to prescribe outside the phase 4 study, participation in these trials helps researchers continue to learn more about the treatment. 

In an observational study, investigators assess health outcomes in groups of participants according to a protocol or research plan. Participants may receive interventions, which can include medical products or procedures as part of their routine medical care, but participants are not assigned to specific interventions by the investigator.

In a natural history study, investigators follow a group of people over time who have, or are at risk of developing, a specific medical condition or disease. These studies collect information to determine how the medical condition or disease develops and progresses. Information collected may be used later to develop and test interventions.

This article was written with contributions from Amy Madsen, marketing communications manager for MDA’s research program, and in consultation with Amy Bartlett, NeuroNEXT clinical research manager at The Ohio State University Wexner Medical Center’s Department of Neurology


Editor's note: Read Trial Run: Is participating in a clinical trial right for me to learn more about important considerations before choosing to participate in a clinical trial. Also read Clinical Trials: Preparation Tips and Questions to Ask.

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