Too much vitamin C can give you headaches, side or lower back pain, diarrhea, nausea. If a supplement doesn't seem to be working for whatever you're taking it for, I'd ease off it a bit? Sometimes it needs other supplements with it too.

I take C for my asthma/respiratory health but only take 500 a day (with a multi-vitamin (has more C), B complex, OsCal Ultra calcium (has more C too) and CoQ10. It doesn't seem likely to me that a lot of any one nutrient would be missing from how we're made that it would take science or medicine, this many zillions of years later to figure it out.