According to The Merck Manuals Online Medical Library, the tolerable upper limit for vitamin C is 2,000 mg per day, or 2 g. Anything above this amount could be considered as an overdose.
Common side effects of vitamin C overdose involve the digestive tract. If you consume more than 2,000 mg of vitamin C, you may develop severe gastrointestinal irritation and diarrhea. In addition to significant discomfort, extended episodes of diarrhea or vomiting can lead to dehydration, causing extreme thirst, fatigue, low urine output and reduced blood pressure. Diarrhea and vomiting can also cause additional mineral imbalances in the body.
Vitamin C is broken down by the body into a substance called oxalate. Some oxalate is secreted in urine, however high urinary oxalate levels may cause formation of deposits, known as kidney stones. High vitamin C intake is associated with an increased risk of oxalate kidney stones -- accounting for approximately 80 percent of all kidney stones.