Echinacea is purported to be an immunostimulating agent capable of preventing or relieving a variety of conditions, such as colds and other respiratory infections, chronic arthritis and chronic fatigue syndrome. However, clinical efficacy has not been proved in randomized controlled trials. Grimm and Müller conducted a double-blind, placebo-controlled study of the effects of a fluid extract of Echinacea purpurea to determine whether it reduces the incidence and severity of the common cold.

The 108 subjects in the eight-week study had a history of at least three respiratory infections or colds during the preceding year. They were randomly assigned to receive 4 mL of the echinacea extract or placebo twice daily. Follow-up visits were four and eight weeks after beginning the study. In addition, the subjects were instructed to seek follow-up if they noted the onset of cold symptoms. If a cold occurred, patients were evaluated every two to three days, and the severity of the cold was rated as mild, moderate or severe. The main efficacy measures were the incidence and severity of colds and other respiratory infections.

At the end of the eight-week study, the number of subjects who developed colds or other respiratory infections was similar in the echinacea and placebo groups. A cold developed in 35 (65 percent) of the 54 subjects who received echinacea and in 40 (74 percent) of those who received placebo. No significant differences between the two groups were noted in the mean number and duration of colds.

The authors conclude that the fluid extract of echinacea does not significantly decrease the incidence or severity of colds and other respiratory infections.