One of the more well-known side effects of high dose oral vitamin C is diarrhea and other gastrointestinal issues like pain, nausea and indigestion. If a person continues to ingest vitamin C at a dose that loosens stool, he/she is at risk of becoming nutritionally deficient. 

Vitamin C is metabolized to oxalate, which can contribute to the formation of calcium oxalate stones in the kidneys. Your genes, dosage, type, immune system and metabolism all play a role. 

High dose vitamin C can cause an imbalance in the person’s pro-oxidation and antioxidation status. At high doses, Vitamin C causes more oxidation, whereas, in lower doses, Vitamin C is a potent antioxidant. 

Vitamin C increases absorption of iron in the cells. High and low levels of vitamin C may impact iron levels in the body.