Vitamin C should not be taken with other antioxidants, for example, selenium and coffee as the oxidized form of vitamin C is transported by GLUT transporters. Blocking oxidation of ascorbic acid inhibited the enhanced vitamin C uptake. Other antioxidants taken with vitamin C could reduce absorption of oxidized vitamin C. Vitamin C should also not be taken with carbohydrates or sugar as glucose could competitively inhibit the transport of vitamin C by glucose transporters. Fat soluble antioxidant supplements, such as vitamin E and carotenoids should not be taken in much more than RDA amounts as fat soluble antioxidants could reduce dedydroascorbic acid to ascorbic acid throughout the day. A selling point of vitamin E has been that vitamin E reduces oxidized vitamin C but as it turns out this is a very large negative.
Vitamin C is taken once a day. The half life of vitamin C is 16 to 20 days. No more than 1 gram in total of vitamin C should be taken a day. Fluctuating vitamin C levels from very high dosages of vitamin C may not be optimal. The huge overlooked fact about vitamin C absorption is taking vitamin C with other antioxidants can very negatively affect the absorption of vitamin C especially given chronic illnesses.
I am not recommending this but some iron sulfate taken with vitamin C could increase levels of dehydroascorbic acid. As vitamin C increases iron absorption and as iron is required by the gut supplemental iron would still have to be taken away from vitamin C.