Any extra vitamin C will simply be flushed out of your body in your urine. For adults, the recommended dietary reference intake for vitamin C is 65 to 90 milligrams (mg) a day, and the upper limit is 2,000 mg a day. Although too much dietary vitamin C is unlikely to be harmful, megadoses of vitamin C supplements may cause: 1  Diarrhea. 2  Nausea. 3  Vomiting. 4  Heartburn. 5  Abdominal bloating and cramps. 6  Headache. 7  Insomnia. 8  Kidney stones.