Too much vitamin C, a water-soluble vitamin, is not usually harmful; however, large doses of supplements containing it sometimes lead to gastric upset, headaches and insomnia, according to Mayo Clinic. The recommended daily reference intake for vitamin C is 65 to 90 milligrams, but the upper limit is 2,000 milligrams.
Most people get enough vitamin C from a cup of strawberries or a piece of citrus fruit. The body flushes any excess amounts out in the urine. Exceeding the upper limit is usually only a problem with supplements. Gastric disturbances associated with large doses of vitamin C include diarrhea, vomiting, heartburn and nausea. Too much vitamin C causes abdominal cramps and bloating in some people, explains Mayo Clinic.
Consumers who take large doses of vitamin C often use it for its antioxidant properties, notes the Merck Manual Consumer Version. Free radicals are thought to contribute to memory loss, cancer, cataracts, lung disorders and the common cold. As of 2015, it is unclear if large doses of vitamin C provide any benefits for these disorders, but the strongest evidence is of its preventative properties in the formation of cataracts. A high dose of vitamin C is also the treatment for scurvy, a health condition related to low levels of the vitamin in the human body.