Thanks - here is what I'm currently thinking ...

Like most fatigued chronically-ill people my mineral chemistry is all screwed up. In particular I have high calcium, low other things including magensium. This is typical of someone suffering from fatigue.

Now my guess is that the Buffered Vitamin C contains calcium in a highly assimilable form, which my body is absorbing. Unfortunately I'm already high in calcium.

This is having a neurological impact. I'm getting calcium overload in the brain and as a result feeling blue.

Its a very definite effect which I have noticed many times over several months. The only other thing is it could be some kind of detox reaction to the Vit C itself, but I really doubt it as it is like no other detox reaction I've experienced, and I've experienced many detox reacations. Rather this is a specific neurological side-effect.