Safety and side effects

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
• Stomach cramps or bloating
• Fatigue and sleepiness, or sometimes insomnia
• Headache
• Skin flushing

In some people, oral vitamin C supplements can cause kidney stones, especially when taken in high doses. Long-term use of oral vitamin C supplements over 2,000 milligrams a day increases the risk of significant side effects.

Tell your doctor that you're taking vitamin C supplements before having any medical tests. High levels of vitamin C might interfere with the results of certain tests, such as stool tests for occult blood or glucose screening tests.