Ascorbic acid, also known as vitamin C, is well known for its ability to strengthen the immune system, act as an antioxidant, and play a crucial part in the production of collagen. Many people think that “more is better” and consider it to be a harmless nutrient. But in the pharmaceutical industry, we understand how important it is to eat a balanced diet and that even healthy substances can be harmful if taken in excess. Megadoses can be harmful, especially to the kidneys, even though the body normally eliminates excess vitamin C.
Oxalate formation is the main issue with vitamin C overdose and kidney health. Oxalate is one byproduct of the body’s metabolism of vitamin C (NIH, 2021). As a naturally occurring substance, oxalate is normally removed from the body in urine. However, when large amounts of vitamin C are consumed, the production of oxalate can significantly increase.
This elevated oxalate level becomes problematic because oxalate can bind with calcium to form calcium oxalate crystals. These crystals are the most common component of kidney stones (National Kidney Foundation, 2025). While not everyone who takes high doses of vitamin C will develop kidney stones, individuals with pre-existing kidney conditions, a history of kidney stones, or certain genetic predispositions are at a significantly higher risk (Ferraro, P. M. et al. 2016)
Calcium oxalate kidney stone formation can lead to:
Intense Pain: Kidney stones can cause intense pain in the side, belly, back, and groin as they attempt to pass through the urinary tract.
Blockage of the Urine: Stones can block the flow of urine, leading to painful kidney swelling (hydronephrosis) and potentially kidney damage if left untreated.
Infection: Trapped urine can become a breeding ground for bacteria, resulting in urinary tract infections (UTIs) and even more serious kidney infection.
Kidney Damage: Recurrent or chronic kidney stones have the potential to cause scarring and irreversible damage to kidney tissue, potentially impaired kidney function in the future. (Cleveland Clinic. 2023)
Other than the development of kidney stones, there is another, albeit lesser known, condition referred to as oxalate nephropathy. Most notably this happens when too much calcium oxalate crystals precipitate into the kidney tubules and cause acute kidney injury or chronic kidney disease (Boyer, A. J. et.al 2018) Although rare, the condition is more likely to occur in an individual with some underlying kidney failure or in the event of extremely high intravenous doses of vitamin C.
The recommended daily allowance (RDA) for vitamin C for most adults is 75-90 mg [7]. The tolerable upper intake level (UL) for adults is set at 2,000 mg (2 grams) per day [7]. Consuming doses consistently above this UL significantly increases the risk of adverse effects, including gastrointestinal upset (nausea, diarrhea, abdominal cramps) and, more importantly, the potential for kidney complications.
It is crucial for consumers to understand that supplements are not always benign. Before embarking on high-dose vitamin C regimens, especially for prolonged periods, individuals should consult with a healthcare professional. (Canales, B. K. 2018) This is particularly important for those with a history of kidney problems or other chronic health conditions. While vitamin C is vital for health, responsible intake within recommended guidelines is key to harnessing its benefits without inadvertently causing harm to the body’s vital organs.
References:
National Institutes of Health (NIH) – Office of Dietary Supplements. (2021). Vitamin C Fact Sheet for Health Professionals. Retrieved from https://ods.od.nih.gov/factsheets/VitaminC-HealthProfessional/
National Kidney Foundation. (n.d.). Kidney Stones. Retrieved from https://www.kidney.org/atoz/content/kidneystones
Ferraro PM, Curhan GC, Gambaro G, Taylor EN. Total, Dietary, and Supplemental Vitamin C Intake and Risk of Incident Kidney Stones. Am J Kidney Dis. 2016 Mar;67(3):400-7. doi: 10.1053/j.ajkd.2015.09.005. Epub 2015 Oct 14. PMID: 26463139; PMCID: PMC4769668.
Cleveland Clinic. (2023). Kidney Stones: Symptoms, Causes, Treatment & Prevention. Retrieved from https://my.clevelandclinic.org/health/diseases/15604-kidney-stones
[5] Boyer, A. J., Stauffer, J. M., & Saland, J. M. (2018). Oxalate nephropathy and vitamin C intake. Clinical Kidney Journal, 11(3), 405-408.
[6] Canales, B. K., & Eisner, B. H. (2018). Vitamin C, hyperoxaluria, and kidney stones. Current Opinion in Urology, 28(4), 369-373.
[7] Mayo Clinic. (2023). Vitamin C (ascorbic acid). Retrieved from https://www.mayoclinic.org/drugs-supplements-vitamin-c/art-20363932
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