Vitamin C is water-soluble, so excess is mostly passed in urine, which makes serious toxicity uncommon. However, large doses (well above normal dietary amounts) often cause diarrhoea, nausea, bloating and cramps. In susceptible people, high intake over time can increase the risk of certain kidney stones.
For most healthy adults, the vitamin C in a balanced diet plus a standard supplement is plenty. Megadoses have not been reliably shown to prevent colds, though they may modestly shorten cold duration in some people.
If you have kidney disease, a history of kidney stones, or conditions affecting iron levels (vitamin C increases iron absorption), check with a doctor or pharmacist before taking high-dose supplements. Stop and seek advice if high doses cause persistent digestive symptoms.