What is a Phase 0 Clinical Trial?

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When a drug is developed for use in a particular disease condition, it undergoes several stages of testing in animal models before being tested on select human volunteers. A phase 0 trial is the first trial where a proposed drug is used on a very small number of human volunteers.

Keys points about Phase 0 trials:

The Phase 0 trial is the first human clinical trial of a new, untested drug.

Due to the unknown risk posed by the new drug, the sample of volunteers selected for this phase is very small, usually between 10 and 12 individuals.

At this early stage of testing, the people chosen are usually healthy volunteers rather than individuals suffering from the condition the drug is indicated for.

Due to the unknown risks surrounding use of the drug, very low amounts of the drug are used for minimal durations of less than seven days, to minimize any adverse side effects that exposure to the drug may cause. Low doses that were found to be nontoxic in the animal studies are used.

The aim of Phase 0 trials is not to test the therapeutic effects but to check that the drug behaves as expected in humans and to gather preliminary data regarding what the drug does to the body and what the body does to the drug.

The Phase 0 trial is also used to rank candidate drugs and select which ones should progress to Phase I trial. Phase 0 trials enable such decisions to be based on human studies rather than what can sometimes be unreliable animal data. Drugs that progress to Phase 1 trials are tested in escalating doses to help establish a safe dosing regimen.

A Phase 0 trial is also called a Pre-phase 1 trial or a "proof-of-concept" trial.

A Phase 0 trial is needed to increase the safety and success of new drug trials.

Further Reading

Last Updated: Mar 13, 2023

Dr. Ananya Mandal

Written by

Dr. Ananya Mandal

Dr. Ananya Mandal is a doctor by profession, lecturer by vocation and a medical writer by passion. She specialized in Clinical Pharmacology after her bachelor's (MBBS). For her, health communication is not just writing complicated reviews for professionals but making medical knowledge understandable and available to the general public as well.


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