If you insist on taking such huge doses of vitamin c, then at least space them eveny over several doses a day(ie. 2 grams every 3 hours) rather than taking 9 or 18 grams at a time. Also take it with plenty of water. Regular acidic vitamin c might be too acidic in such large doses, so using calcium and or magnesium ascorbate(or perhaps some would say ester c is the best) might be a good idea.