Echinacea

The leaves, roots, and other parts of echinacea (a purple daisy belonging to the aster family) are sold as capsules, tinctures, juices, and teas. Nine kinds grow in the U.S., the most common being Echinacea purpurea. Echinacea extracts have been used medicinally for centuries and are widely used in Europe as remedies for colds and flu.

Claims, purported benefits: Prevents/cures colds, boosts immunity.
Evidence: Echinacea has been extensively studied, but with conflicting results. In some studies involving humans, echinacea seems to have no effect on colds or immunity; in others, it seems to help. In a 2006 German review of studies, some Echinacea purpurea preparations shortened the duration of colds in adults, but there was no evidence that other preparations were effective. A 2007 meta-analysis suggested that the herb can reduce the chance of catching cold and shorten the duration or severity of colds.

Bottom line: Some chemicals in echinacea may help stimulate the immune system, but the benefits against the common cold are not confirmed by scientific research. No one knows what the active ingredient is, or what dosage, if any, is best. It may depend on the plant species and/or plant part. Preparations vary widely. As with all unregulated herbal products, what you buy may not contain much, if any, echinacea, and products available in the U.S. lack testing. Moreover, some researchers think echinacea could have an adverse effect on T-4 cells, an important component of the immune system; little is known about toxicity. Some people should not take echinacea: those allergic to daisies, marigolds, and plants in the aster family; those with diseases such as lupus, rheumatoid arthritis, tuberculosis, and multiple sclerosis; those who are HIV-positive; pregnant or nursing women and small children.