Mineral deficiencies. Mega dose vitamin c can actually be dangerous. It is true that vitamin c is water soluble & you will excrete any vitamin c that you don't use. However, in order to excrete vitamin c in your urine, you must also excrete minerals as well. Therefore, excess vitamin c can lead to mineral deficiencies, especially a copper deficiency. Be very cautious in megadosing any vitamin or mineral or amino acid.
Laxative effect. Vitamin c is extremely safe. Excessive doses (usually over 5000 mg a day) can cause a laxative effect. Vit. C can interact with certain medications, esp. Coumadin (warfarin) (which interacts with everything!). Concerns that high doses of vit c can cause kidney stones are unproven, in fact there is no good evidence of harm from high doses of c. See http://lpi.Oregonstate.Edu/infocenter/vitamins/vitaminc/.