In over 25 years of experience, he has found that healthy people can take 10-15 g/day before suffering from diarrhoea, reaching 30-60 g/day for people with colds, and 200 g/day for those with serious infectious diseases.
Reports of vitamin C raising blood uric-acid levels - or 'rebound scurvy' - have proved to be unfounded, as is the supposed increased risk of kidney stones - which, although postulated in theory, have never been found in practice. As a recent review of all the evidence puts it: "Numerous studies of vitamin C supplementation have provided no pattern of evidence to support concerns about safety other than occasional gastrointestinal upset" (Am J Clin Nutr, 2005; 81: 736-45). Neither has anyone yet found a dose above which the vitamin becomes unsafe (Nutr Rev, 1999; 57: 71-7).
Vitamin C boosts iron uptake, so individuals with the genetic iron-overload problem called 'haemochromatosis' should take extra vitamin E to counteract the effects of vitamin C.