I've never heard anything like that..
I often take 5,000 or more if I feel a cold coming on or unwell.
I don't take Vit C constantly though..

When I had my RSI/TOS injuries I upped my Vit C along with other supplements.


This is what I found searching for info from the book - a snippet re Vit C
[ Doses of vitamin C in excess of 500 mg daily may interfere with blood sugar readings (causing them to appear erroneously low). Large doses of vitamin C can actually raise blood sugar, cause kidney stones, and even impair nerve function ]
http://www.diabetes-book.com/cms/art...ce-facn-fccws-