The National Center for Complementary and Alternative Medicine (NCCAM), defines complementary and alternative medicine as diverse medical and health care systems, practices, and products that are not presently part of conventional medicine. Some scientific evidence exists regarding complementary and alternative therapies, but for most there are many questions in need of answers through well-designed scientific studies.

Complementary medicine is used along with conventional medicine (for example, using massage and drug therapy to reduce the discomfort of fibromyalgia).

Alternative medicine replaces conventional medicine (for example, using a special diet to treat cancer instead of conventional cancer treatments such as chemotherapy, radiation, and surgery).

Integrative medicine combines mainstream medical therapies with complementary and alternative therapies for which there is some high-quality scientific evidence of safety and effectiveness.