Experimental drugs and treatments are tested and evaluated in four phases of study, or trials. These phases are explained in detail later in this chapter. In preclinical studies, scientists gather information and evidence from laboratory research, animal testing, and human observational trials indicating that a treatment shows promise and that it would provide a benefit that current medications don't.

In clinical trials, volunteer subjects work with a research team of doctors, nurses, and other health professionals at private research facilities, teaching hospitals, AD research centers, or doctors' offices. Researchers and volunteers follow a protocol — a study plan that determines who may participate in the trial (people with early-stage AD or those already taking medication, for example); the schedule of tests, treatments, and procedures involved; and the length of the study. The protocol is designed to assure the health and well-being of participants as it answers questions about the treatment.

Most clinical trials randomly assign participants to a study group. One group, the test or control group, is given the experimental drug or treatment. Other groups may receive a different drug or a placebo, an inactive substance that looks like the study drug. Only by comparing the groups can researchers be sure that changes among patients in the test group are caused by the treatment, not something else.