Barrett B, Vohmann M, Calabrese C. Echinacea for upper respiratory infection. J Fam Pract 1999 Aug;48(8):628-35.
Lindenmuth GF, Lindenmuth EB. The efficacy of echinacea compound herbal tea preparation on the severity and duration of upper respiratory and flu symptoms: a randomized, double-blind placebo-controlled study. J Altern Complement Med 2000 Aug;6(4):327-34.
