Echinacea is a herbal supplement that is commonly used either at the first signs of sickness (in an attempt to accelerate the rate of recovery) or daily as a preventative supplement in persons who are sick frequently (in hopes of reducing the frequency of which they get sick). The term 'echinacea' refers to a genera of plants, and a few species in this family including purpurea and angustifolia are desired due to their alkylamide content (seen as the active ingredients). Overall, echinacea appears to be somewhat effective for fighting off sickness and accelerating the rate of recovery in sickness but both of these claims are highly variable. There are trials suggesting remarkable recovery rates, and there are trials suggesting no benefits whatsoever. When looking at meta-analyses on the topic, there appears to be a positive and protective effect of echinacea on sickness frequency (in those who are frequently sick) and in accelerating the rate of recovery; the effect size, however, is not large.
For dehydrated powders (including encapsulated echinacea) the species of purpuera tends to be used and oral doses are taken upwards of 300mg thrice a day (900mg daily) and 500mg thrice daily (1,500mg daily).
Tinctures of an ethanolic extract of the aerial parts (leaves and stems) appears to be used in the concentration of 2.5mL thrice a day or up to 10mL daily.
There is no much evidence as to whether these are the optimal doses, and studies seem to be very heterogeneous in their benefits due to lack of standardization.