I had an apparently-unusual problem, whereby my system seemed to react poorly to the large-dose monthly Vitamin D pills. The problem was hard to pin down, other than a repeated sense that everything (mentals, sleep, digestive system, ...) suddenly sucked, starting, on each occasion, with the taking of a Vitamin D pill.

So I swore off them, and rely upon the natural (sun) source instead.

Vit.D is not a magic cure for chronic/severe conditions, unfortunately - alone, nothing is - but it services critical physiological functions, without which, everything is that much harder.