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:
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.
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.
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.