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Echinacea is an herb that has been used to treat or prevent symptoms related to illnesses such as common colds and the flu. To date, studies have revealed mixed results as to whether echinacea prevents or shortens illness.

The species of plants known as Echinacea are flowering perennials that are native to the East and Midwest regions of the United States. There are nine species, and three are used in supplements: Echinacea purpurea, Echinacea angustifolia and Echinacea pallida. Echinacea is sold under various trade names and is one of the most popular supplements in the United States.

The National Institutes of Health (NIH) has conducted two studies that have revealed no link between taking echinacea regularly (e.g., on a daily basis) and the prevention of colds and other infections. However, other studies have indicated that echinacea may provide benefits to those with upper respiratory infections. The NIH and other organizations are continuing to explore the potential link between echinacea use and a reduction in the severity and duration of illnesses such as colds and the flu.

In most people, echinacea does not cause side effects. However, some people may experience an allergic reaction, the most serious of which is anaphylactic shock, a life-threatening, whole-body allergic reaction associated with a collapse of the circulatory system. Other potential side effects include gastrointestinal symptoms, dizziness, nausea, swelling and rash.

People with certain allergies should not take echinacea. This is particularly true of people who are allergic to plants in the Asteraceae or Compositae family. Examples of such plants include ragweed, chrysanthemums, marigolds and daisies.

Few studies have been performed on the potential interactions between echinacea and medications, supplements or other herbs. In rare cases, echinacea has been linked with liver inflammation (hepatitis). As a result, patients taking certain medications are urged to avoid taking echinacea. It is important to tell physicians and other medical professionals about all supplements and herbs that a patient is taking.