Herbal books in North America often state that Echinacea will lose its effectiveness after ten days of continuous administration. This Herban Legend is based on the mistranslation of a German research article (1). The article contains a graph showing the immune activity of white blood cells. The activity appears to peak at day five, and decline back toward the baseline by day ten. The text of the article explains that the echinacea was only given for five days, a fact that eluded the originator of this Legend, which dates to the late 1980s. Other research has shown that echinacea continues to build immunity for as long as six weeks (2).
See full article
(1) Jurcic K, Melchart D, Holsmann M, Martin P, et al. “Zwei probandenstudien zur stimulierung der granulozyphagozytose durch echinacea-extract-haltige präparate.” Zeitschrift för Phytotherapie 1989;10:67-70
(2) Coeugniet E, Kuhnast R. Recurrent candidiasis. Adjuvant immunotherapy with different formulations of Echinacea. Therapiewoche 1986;36:3352-8 [in German].