What are clinical trials?

Clinical trials are studies carried out in human volunteers over a period of 10-15 years to investigate the safety and/or efficacy of a medicine. The Food and Drug Administration (FDA) describes the following phases:

  • Phase 0

Tests a few small doses (less than 1%) of the therapeutic dose of an investigational drug in a few volunteers. The trial takes no more than seven days, and the greatly reduced dose would ensure the absence of toxic effects. Its main goal is to quickly establish whether an agent will work as desired in humans and show whether a drug's pharmacokinetics and pharmacodynamics warrant continued exploration.

The process will not demonstrate a therapeutic effect, nor will it replace the dose escalation, safety, and tolerance studies required in Phase I, but it should quickly weed out ineffective drugs.

  • Phase I

Assesses the drug's most frequent (and serious) adverse events and how the drug is metabolised and excreted.

The number of subjects (either healthy volunteers or people with the disease/condition) ranges from 20 to 100. Phase I takes several months.

  • Phase II

Provides preliminary data on the biological activity of a drug. Safety continues to be evaluated and short-term adverse events are studied.

Up to 300 subjects (volunteers with the disease/condition) are studied. Phase II takes between several months to two years.

  • Phase III

Continues assessing effectiveness and adverse reactions.

It is carried out in 300 - 3,000 volunteers with the disease/condition from different human populations. Different dosages and the combination with other drugs can be assessed. This phase takes between one and four years.

  • Phase IV

Takes place once the FDA authorises a drug for marketing. Further information on drug's safety, efficacy and/or optimal use will be assessed in pharmacovigilance.

Some phase IV drugs (e.g. from ClinicalTrials.gov) may also be in postmarketing surveillance, and might be used for conditions different from the ones for which the drug has been originally approved for. Several thousands volunteers who have the disease/condition are studied.

This FDA infographic gives a summary of the different phases in a clinical trial.