(See also Overview of Dietary Supplements.)
Echinacea, a North American wildflower, contains a variety of biologically active substances.
Studies of echinacea's role in preventing and/or treating the common cold are inconsistent. The largest factor contributing to inconsistency is the variability of plant preparations (including different plant parts and species) and ultimately composition of the supplement. According to a 2006 Cochrane review, the aerial parts of the plant might be effective for the early treatment of colds; also, the other preparations of echinacea, might have a role in prevention of colds (1). However, more meticulous randomized, controlled trials are necessary.
Most adverse effects are mild and transitory; they include dizziness, fatigue, headache, and GI symptoms. No other adverse effects are known. Theoretically, echinacea is contraindicated in patients with autoimmune disorders, multiple sclerosis, AIDS, tuberculosis, and organ transplants because it may stimulate T cells. Echinacea inhibits some cytochrome P-450 enzymes and stimulates others; it can therefore potentially interact with drugs metabolized by the same enzymes (eg, anabolic steroids, azole antifungals, methotrexate). Allergic reactions are possible in patients with allergies to ragweed, chrysanthemum, marigold, daisies, or related allergens.