Over 30 clinical studies and in vitro studies have been published about Echinacea to date. Though clinical data are limited, E purpurea may decrease symptom severity of the common cold and may decrease the number of colds when taken prophylactically, and E pallida root extract may decrease the duration of symptoms of the common cold. More conflicting data exist about Echinacea angustifolia, and while some clinical data suggest it may decrease symptom severity of the common cold, some show it may not.