Echinacea, a North American wildflower, contains a variety of biologically active substances.
(See also Overview of Dietary Supplements.)
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.
A 2014 Cochrane review of 24 studies (4631 participants) evaluated a variety of different echinacea products to prevent or treat colds. No benefit was shown for prevention, although a post hoc pooling of prevention studies implied a 10 to 20% relative risk reduction. The authors concluded some products had weak benefit (1). A 2019 systematic review and meta-analysis of randomized trials assessed the impact of echinacea on prevention, duration, and safety for upper respiratory tract infections. There was a significant effect on prevention but no impact on symptom duration and no safety risk (2). However, a limitation was heterogeneity of included studies.
Most adverse effects are mild and transitory; they include dizziness, fatigue, headache, and gastrointestinal 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. Allergic reactions are possible in patients with allergies to ragweed, chrysanthemum, marigold, daisies, or related allergens.
Karsch-Volk M, Barrett B, Kiefer D, et al: Echinacea for preventing and treating the common cold. Cochrane Database Syst Rev (2) CD000530, 2014. doi: 10.1002/14651858.CD000530.pub3.
David S, Cunningham R: Echinacea for the prevention and treatment of upper respiratory tract infections: a systematic review and meta-analysis. Complement Ther Med 44:18-26, 2019. doi: 10.1016/j.ctim.2019.03.011.
The following is an English-language resource that may be useful. Please note that THE MANUAL is not responsible for the content of this resource.
National Institutes of Health (NIH), National Center for Complementary and Integrative Health: General information on the use of echinacea as a dietary supplement