Clinical trial

A clinical trial is a research study conducted with human volunteers to evaluate whether a new medical treatment, device, or intervention is safe and effective before it becomes available for general use. Clinical trials follow a structured, carefully regulated process designed to generate reliable evidence while protecting the rights and wellbeing of the people who participate, forming the foundation of evidence-based medicine as it is practiced in hospitals and clinics around the world today.

The Phases of Clinical Trials

Clinical trials for new drugs typically proceed through a series of phases, each designed to answer different questions and each building on the data gathered from the phase before it. Phase one trials involve a small group of healthy volunteers or patients and focus primarily on safety and appropriate dosing, phase two trials involve a somewhat larger group of patients and begin evaluating effectiveness alongside continued safety monitoring, and phase three trials involve much larger groups, often across multiple research sites, to confirm effectiveness, monitor for less common side effects, and compare the new treatment against existing standard care.

Randomization and Control Groups

Many clinical trials randomly assign participants to either receive the experimental treatment or a comparison, which may be an existing standard treatment or, when ethically appropriate, a placebo that resembles the treatment but contains no active ingredient. Randomization helps ensure that differences in outcomes between groups can be attributed to the treatment itself rather than to pre-existing differences between the people who happened to receive each option, which is part of why randomized controlled trials are generally considered the strongest type of evidence for establishing whether a treatment actually works. Observational studies, which simply track outcomes among people already receiving different treatments in ordinary practice, remain valuable for generating early hypotheses, but they are more vulnerable to confounding factors that randomization is specifically designed to address.

Blinding

Blinding, in which participants, the researchers administering treatment, or both are kept unaware of who is receiving the experimental treatment versus the comparison, helps prevent expectations from influencing either how participants report their symptoms or how researchers interpret and record results. A trial in which only participants are unaware of their assignment is called single-blind, while a trial in which both participants and the researchers directly interacting with them are unaware is called double-blind, generally considered a stronger design because it removes an additional source of potential bias.

Informed Consent and Ethics

Before joining a clinical trial, participants must go through an informed consent process in which researchers explain the trial’s purpose, procedures, potential risks and benefits, and the participant’s right to withdraw at any time without penalty. Independent ethics committees, often called institutional review boards, review and approve trial protocols before they begin and continue monitoring ongoing trials to ensure participant safety and rights remain protected throughout the entire course of the study.

From Trial Results to Approval

Once a series of clinical trials demonstrates that a treatment is both safe and effective, researchers and sponsoring organizations submit the collected data to regulatory agencies, which conduct an independent review before deciding whether to approve the treatment for general use. Even after approval, many treatments continue to be monitored through a further stage sometimes called phase four or post-marketing surveillance, which can detect rare side effects that only become apparent once a treatment is used by a much larger and more diverse population, over a much longer period of time, than was included in the original trials.

Who Can Participate

Each clinical trial defines specific eligibility criteria, known as inclusion and exclusion criteria, that determine who can participate, often based on factors such as age, the specific diagnosis or stage of a condition, prior treatments received, and overall health status. These criteria serve two purposes: they help ensure participants are reasonably likely to benefit from and safely tolerate the experimental treatment, and they help researchers study a more consistent population so that differences in outcomes can be more clearly attributed to the treatment itself rather than to wide variation among participants.

Historically, clinical trials have sometimes underrepresented certain groups, including women, older adults, and various racial and ethnic populations, which can limit how confidently trial results apply to those groups once a treatment reaches general use. Many research institutions and regulatory agencies now actively work to improve the diversity of trial participants, recognizing that a treatment’s safety profile and overall effectiveness can sometimes differ meaningfully from one population to another.

Clinical Trial Registries

Public clinical trial registries allow researchers, clinicians, and the public to search for ongoing and completed trials, view their design and eligibility criteria, and in many cases see summary results once a trial concludes. These registries serve an important transparency function, helping prevent the selective reporting of only favorable results while allowing patients and their doctors to identify trials that a patient might be eligible to join, particularly for conditions with few existing effective treatment options available through standard care.

FAQs

Why do clinical trials use randomization?
Randomly assigning participants to treatment groups helps ensure that differences in outcomes can be attributed to the treatment itself rather than to pre-existing differences between participants, making randomized trials a particularly strong form of evidence.

What is the difference between single-blind and double-blind trials?
In a single-blind trial, only participants are unaware of their treatment assignment, while in a double-blind trial, both participants and the researchers directly interacting with them are unaware, which removes an additional source of potential bias.

Editorially reviewed by Specialty Digest Editorial TeamLast reviewed November 2, 2026Researched and drafted with AI assistanceReport an issue