Clinical trials are conducted in phases. Each phase is designed to answer specific scientific questions:
Phase 1 trials are conducted to find out what is the best dosage of a new intervention that can be given safely without serious side effects. The first few people in the study often get a low dose of the treatment and are watched very closely. If there are no side effects or only minor side effects, the next few patients may receive a higher dose. This process is repeated until doctors find the dose that's most likely to work while having an acceptable level of side effects.
Once the best dosage and dose schedule [how often a drug is given (i.e. once per day, twice per day, once per week, etc.)] are found, the trial may move into phase 2
Phase 2 trials are conducted to see if an intervention is effective (does it work?). Participants are given the optimal dose of a new intervention that is thought to be reasonably safe. This dose and schedule are based on results from phase 1 trials. If the intervention appears helpful, the study may move into phase 3.
Phase 3 trials are conducted to see if the new intervention is better than the current standard of care treatment. Phase 3 study participants are assigned to either receive the new intervention (called the study group) or the standard of care intervention (called the control group) so that researchers can compare which intervention works best.
Participants have a 50:50 chance (like a flip of a coin) of ending up in one group or the other. Sometimes neither you nor your doctor will know which treatment you are receiving until the study ends; this is called blinded randomization.