“The intake of up to 1 g Vitamin C per day, in addition to the intake with your normal diet should not give any harmful side effects. With an intake of 3 to 4 g per day, gastrointestinal problems, such as diarrhoea can temporarily occur.
There are some groups of people who have an increased risk of harmful side effects due to increased Vitamin C intake: people with kidney damage, people with a predisposition for urinary or kidney stones or with disorders in the absorbation of food iron. However, it is also tolerable for them to take up to 1 g per day, without any harmful side effects.”