When taken at appropriate doses, oral Vitamin C supplements are generally considered safe. Taking too much Vitamin C can cause side effects, including: Nausea, vomiting and diarrhea; Heartburn; Increase urination; Stomach cramps or bloating; Fatigue and sleepiness, or sometimes insomnia; Headache; Skin flushing.
In some people, oral Vitamin C can cause kidney stones, especially when taken in high doses.