Vitamin C has a strange double reputation: some sources claim it helps "flush" kidney stones, others warn that it causes them. The UK evidence is clearer than the headlines: moderate doses are safe and useful, megadoses are a stone risk.

Why high-dose vitamin C is risky for stones

The body converts excess vitamin C (ascorbic acid) into oxalate. About 80% of UK kidney stones contain calcium oxalate, so anything that raises urinary oxalate raises stone risk in susceptible people. Studies have shown that men taking 1,000 mg or more of vitamin C daily had roughly double the kidney stone risk compared with those taking under 90 mg.

Does vitamin C help prevent kidney stones?

At food-level doses (60–90 mg), vitamin C is part of a healthy diet and there is no evidence it raises stone risk. There is also no good evidence that vitamin C supplements prevent stones. The strongest UK-recognised stone-prevention measures are simpler:

  • Drink 2.5–3 litres of fluid a day (NICE NG118).
  • Reduce salt to under 6 g a day.
  • Maintain a moderate, not megadose, calcium intake from food (700 mg/day for adults).
  • Limit oxalate-rich foods only if your stones are calcium-oxalate and your renal team advises it.

What the NHS recommends

The UK Reference Nutrient Intake for vitamin C in adults is 40 mg per day. Most multivitamins provide 60–90 mg, which is plenty. The NHS does not recommend taking more than 1,000 mg per day — above this, some people develop diarrhoea, stomach pain and a higher kidney stone risk.

Sensible UK dose limits

  • Healthy adults, no stone history: up to 200 mg/day from supplements is a comfortable ceiling.
  • Adults with a calcium-oxalate stone history: stay close to RNI (40–90 mg) and get vitamin C from fruit and vegetables.
  • Adults with CKD: moderate doses only; megadose vitamin C is best avoided.

Bottom line

Vitamin C does not magically prevent kidney stones — and at megadoses it can cause them. At NHS-aligned doses (under 200 mg/day) it is safe for most adults, including most people with a kidney stone history. Hydration, salt reduction and moderate calcium intake do far more to prevent stones than any supplement.