No, have never experienced diarreah or nausea, or any side effect, ever. On the contrary. I've been taking Echinacea for the past 26 years, and so has my teenage son since he was six months old. Every time we feel "funny" as if we're coming down with something, like flu symptoms, e.g., symptoms of sore throat, eyes burning, ear pain, sneezing, etc., we "attack it" right away with It, we take a full dropper every hour on the hour. It works 95% of the time to avoid getting the flu, and the symptoms go away and we don't get to get sick at all. The times we did get the flu although we got it, we don't feel sick at all, we feel fine, just that the mucuos is still there. For it to work, though, we take alot, as I said, a full dropper every hour (sometimes every l/2 hour) with water or juice 'til the symptoms subside completely. The rest of the time we don't take it at all.