New drugs and devices typically go through four, and sometimes five, phases of clinical testing. Three of these happen before treatments are approved and sold to the public. The fourth and fifth phases, which are types of post-marketing studies, occur after products are more widely released.
Deeper dive
In Phase 1, new treatments that have been developed in test tubes and vetted in animals are given to human patients for the first time. These studies typically enroll very small numbers of patients who take a new treatment for a limited period of time. The population size may range from 12 to 100 people. The goal of phase 1 studies is to demonstrate safety. They often use different drug dosages to measure responses and side effects.
Once basic safety is demonstrated, new treatments enter phase 2 studies. In phase two, researchers give drugs to larger groups for longer periods of time. The population size typically includes several hundred volunteers, though some phase 2 trials may be larger or smaller depending on the condition and the treatment.
The goal of phase 2 studies is to demonstrate efficacy, or that a drug works as doctors hope it will. Side effects and adverse events continued to be monitored in phase 2 trials, as well. Many new treatments fail during phase 2. Only about 18 percent of new treatments succeed in phase 2 studies, according to a study published in May 2011 in Nature Reviews Drug Discovery.
Phase 3 studies are large and often expensive studies that pit an experimental remedy against an inert placebo or an established treatment for the same disease (see non-inferiority). Phase 3 studies will always involve at least several hundred participants, but more often they involve several thousands or even tens of thousands.
Phase three studies are designed to evaluate how well a drug will work in the real world. Phase 3 studies also sometimes shed light on the patient population that’s mostly likely to benefit from a new therapy. Regulators rely most heavily on data from phase 3 studies when they weigh whether a drug should be approved.
Phase 4 studies, or post-marketing surveillance, are sometimes required by the FDA when regulators want drug makers to continue to monitor the safety of a drug after approval.
Phase 5, or comparative effectiveness, studies are sometimes undertaken to see which agents in a group of similar treatments work best.