For the average person, the most notable interaction to look out for with vitamin C is its ability to weaken the effects of certain over-the-counter medications. Michels says that vitamin C can increase excretion of certain products into the urine, which quickens the rate at which it leaves the body and shortens the amount of time you feel its effects.
Additionally, people with certain health concerns should take extra care with higher-dose vitamin C supplements:
Individuals with renal health concerns should talk to their health care provider before adding a vitamin C supplement to their regimen, since, according to Michels, ascorbic acid can break down into oxalate (too much of which can affect kidney function).
Those taking blood-thinning medications should also speak with their health care provider before taking a new vitamin C supplement, since it may alter its effects.
Those with very high levels of iron may also want to exercise caution, since vitamin C can increase the amount of iron the body absorbs.
