Phase 0

Also known as a human microdosing study. An optional exploratory trial conducted to speed up the development of promising drugs or imaging agents. It involves administering single subtherapeutic doses of the study drug to a small group of 10-15 subjects to gather preliminary data on the drug’s pharmacokinetics. This phase provides no data on safety or efficacy as the dose is too low to cause any therapeutic effect. The results from this phase help in ranking drug candidates for further development based on their pharmacokinetic parameters in humans.

Source: Wikipedia

Not to be confused with: Preclinical trial
Not to be confused with: Phase I trial