read or heard recently that for cold and flu season while taking vit C to add Vit D. The Vit D helps boost the effect of the Vit C.
Was news to me, but hey whatever works.

I have been using vitamin D 2000 units, but it melts under my tongue, bypasses the "digestive loss" from going through the stomach, goes directly into the bloodstream. Have been feeling great for weeks now.