I was surprised when my GP recommended dosing myself on echinacea in combination with vitamin C to try to shake off winter coughs and colds. It seems there is good science to suggest they might be helpful, especially in combination. This is written up as S. Shah, S. Sander, C. White, M. Rinaldi, C. Coleman. Evaluation of echinacea for the prevention and treatment of the common cold: a meta-analysis. The Lancet Infectious Diseases, 7(7):473-480, July 2007.
Coleman and colleagues reviewed the current literature using an approach called a "meta-analysis" which pools the results of previous trials and examines them statistically. They reviewed 14 trials that studied the effect of echinacea on prevention and treatment of the common cold. Using a method that calculates the odds ratio of incidence of the common cold in the pooled participants, the researchers found that echinacea decreased the odds by 58 per cent and the duration of a cold by 1.4 days. Only one of the studies reviewed looked at a echinacea in combination with vitamin C. This reported a reduction in cold incidence of 86 per cent for the combined dose. Because there was only one study in this category the authors felt they were not able to say with confidence whether the two supplements were better than echinacea alone at fighting off colds.
Echinacea is a family of nine plant species indigenous to North America; the types of echinacea that are most commonly used for medicine are Echinacea angustifolia, Echinacea pallida, and Echinacea purpurea.