Echinacea: An herb that has been claimed to boost the body's immune system and help fight off infections. Echinacea has been widely used to treat the symptoms of upper respiratory tract infections (URIs), including colds and the flu. The herb is derived from the purple coneflower, Echinacea purpurea, a drought-tolerant perennial plant native to North America with large purple flowers surrounding a large cone.

However, the efficacy of echinacea in treating URIs has not been established. In a randomized placebo-controlled trial, echinacea was found not to shorten the duration of a cold or lessen its severity in children (JAMA 2003;290:2824-2830).

The most common side effect of echinacea is an unpleasant taste. Echinacea can cause liver toxicity. It should be avoided in combination with other medications that can affect the liver (such as ketaconazole, leflunomide /Arava, methotrexate /Rheumatrex, isoniazide/Inh/Nizoral). Another side effect is rash.

Echinacea is sold over the counter without a prescription. It is one of the most popular alternative therapies. Americans spend $300 million on it annually.