Before an investigational drug is studied in humans, it has already undergone rigorous studies in laboratories to determine its safety. The study drug is then ready for investigation for the first time in humans.
In phase I/II studies, researchers often give the investigational drug to a small group of participants. This enables them to evaluate the safety of different doses, determine how it should be given, and watch for any side effects.
After this, and depending on the results, a study drug may potentially be investigated further in a larger group of participants, before it can be submitted to a regulatory agency for approval.