Vitamin C, also known as ascorbic acid, is essential for the growth, development, and repair of body tissues. Since the body does not store vitamin C, it must be replenished regularly through foods like oranges, tangerines, strawberries, bell peppers, or supplements when necessary. While vitamin C supplementation is generally safe for most people, high or prolonged doses can lead to several adverse health effects.

Kidney stones

High doses of vitamin C can increase oxalate levels in urine. Oxalate can combine with calcium to form kidney stones. The body's metabolism of excess vitamin C into oxalate is particularly concerning for individuals with a history of kidney stones, chronic kidney disease, or elevated oxalate levels, as it increases the risk of stone formation.

Digestive upset

A common side effect of high-dose vitamin C is digestive upset, including: stomach cramps, nausea, diarrhea, and bloating. Vitamin C is acidic and has osmotic activity; excessive intake draws water into the intestines, irritating the lining and causing discomfort. These symptoms typically manifest when consuming over 2,000 mg per day.

Iron overload

Vitamin C enhances the absorption of non-heme iron from plants. However, high doses can be detrimental for individuals with hemochromatosis, a genetic disorder causing the body to store excess iron. In these patients, high-dose vitamin C exacerbates iron overload, increasing the risk of liver damage, heart problems, and diabetes. Those with iron metabolism disorders should avoid high-dose vitamin C supplements unless advised by a doctor.

Excessive use of vitamin C can lead to harmful health side effects. Illustration created by AI.

Inaccurate test results

Excess vitamin C can interfere with certain medical tests, including blood glucose meters, urine creatinine tests, and assays using oxidation-sensitive reagents. Inaccurate results can impact diagnosis and treatment.

Tooth enamel erosion

Chewable or gummy vitamin C supplements are often acidic, which can erode tooth enamel over time. This erosion leads to tooth sensitivity, discoloration, and an increased risk of cavities. To mitigate this risk, rinse your mouth with water after use and avoid brushing immediately afterward.

Increased oxidative stress

Although vitamin C possesses antioxidant properties, high doses, especially in the presence of free metal ions like iron or copper, can act as a pro-oxidant. This leads to increased oxidative stress, an imbalance between antioxidants and free radicals, which can damage cells. Research indicates that very high concentrations of vitamin C can generate free radicals in the presence of metals, posing a potential risk of cell damage and adverse effects when consumed in large doses.

Recommended daily allowances (RDAs) vary by age, sex, and life stage: men require 90 mg/day, women 75 mg/day, pregnant women 85 mg/day, and breastfeeding women 120 mg/day. The tolerable upper intake level for adults is 2,000 mg/day. Exceeding this amount, especially over extended periods, increases the risk of side effects.

Individuals susceptible to vitamin C side effects include: those with kidney disease, iron overload disorders, diabetes, or those undergoing chemotherapy or radiation therapy. For safe use, adhere to the recommended daily dosage unless otherwise instructed by a doctor. Inform your doctor about all supplements you are taking, especially before surgery or medical tests. Individuals experiencing persistent abdominal pain, blood in their urine, or other signs of kidney stones should seek medical attention.

Anh Ngoc (via Verywell Health)