http://www.pponline.co.uk/encyc/0897.htm
However, says Bauer, 'Echinacea preparations have to be taken at the first signs of a cold'. Don't wait a few days - or even 24 hours - if you expect to get any benefits. And be wary of ingesting Echinacea if you are allergic to plants of the Compositae family, including ragweed, dandelion, feverfew, sneezeweed and tansy as well as cultivated varieties like asters, cornflowers, dahlias, daisies and sunflowers.
Echinacea preparations should be avoided by patients with tuberculosis, leukocyte disorders, collagen problems, multiple sclerosis, HIV infections, and autoimmune diseases, because of lack of clinical experience and research. And, according to Dr Bauer, Echinacea has been associated with worsening of metabolic status in people with diabetes.