positive [ 15 ] [ 16 ] Echinacea purpurea Purple coneflower, and other species of Echinacea Reduce the severity and duration of symptoms associated with cold and flu . inconclusive [ 17 ] Glycyrrhiza glabra Liquorice Hydrastis canadensis Goldenseal Antimicrobial [ 18 ] Hypericum perforatum St. John's wort Antidepressant positive [