Most vitamin C side effects are very mild and typically occur only when a person has taken more than 2,000 mg, which is the maximum daily dosage. Some common side effects of vitamin C are nausea, vomiting, and diarrhea. Other vitamin C side effects that are occasionally reported are headaches, fatigue, and heartburn. Serious side effects from vitamin C are very rare and typically occur only in people who have certain health conditions. People with cancer, kidney stones, and diabetes should take vitamin C supplements under the supervision of a doctor to avoid these serious side effects.
The majority of vitamin C side effects occur when a person takes supplements of vitamin C, but the vitamin can also cause a reaction in people who use the supplement topically. There are some different skin creams and medications that contain vitamin C, and the use of these might cause irritation at the site of application in people with skin allergies. A person who experiences mild side effects from using vitamin C either topically or internally should probably discontinue use of the supplement to see if the problems go away. If they do not, the symptoms may be related to some other health problem that might require medical attention.
People with serious health problems should never take any supplement without first consulting their doctor. This is especially true with vitamin C supplements, because vitamin C side effects can make some existing health problems worse. For example, people with cancer might need to have their vitamin C intake closely monitored because cancer cells seem to absorb an unusually high amount of vitamin C. Scientists are not entirely sure why this occurs, and until more is known, people with cancer are not advised to take the supplements without medical supervision.
Vitamin C might also increase the chances of kidney stones in people who have a history of them. It is also not recommended for a person with existing kidney stones to take any vitamin C supplements because they could make the problem worse. Vitamin C also tends to raise blood sugar in diabetics, and the amount of vitamin C a diabetic person takes each day in addition to what the person absorbs naturally from food should be monitored by a doctor. Pregnant women may also want to be careful not to take more than 120 mg of vitamin C per day. More than this might cause problems for an unborn baby.