At the first symptom of cold or flu (sore throat, fever, general ill feeling) or even exposure to someone else who is sick, start taking elderberry. For me, it has halted the progression of colds that were going around work, which made others (who weren't taking it) very sick, and shortened flu recovery for me the one time I had it 3 years ago, all with no side effects. Gaia is the best. Check the level of elderberry in it - it is more concentrated than the other brand so it is actually more cost effective. It also says it is made from elderberries from Italy which are tracked from origin to point of sale, so I feel this brand is safe.