Clinical research is medical research involving people. There are two types: observational studies and clinical trials.

In an observational study, an investigator observes individuals without manipulation or intervention. For example, researchers may collect data through medical exams, tests, or questionnaires about a group of older adults over time to learn more about the effects of different lifestyles on cognitive health. These studies may help identify new possibilities for clinical trials.

Clinical trials are research studies performed on people that are aimed at evaluating new treatment therapies. They are the primary method in which researchers discover if a new treatment—such as MDMA-assisted psychotherapy—is safe and effective. Often a clinical trial is used to learn if a treatment is more effective and/or has less harmful side effects than an established treatment.