Vitamin C for Kidney Stones

Vitamin C is not a treatment for kidney stones, and no trial has tested it for stone prevention. The real question is whether supplements add risk, and in men they appear to. Men whose total vitamin C intake, from food and supplements, was 1,000 mg or more a day had 41% more stones over 14 years than men below the 90 mg RDA (RR 1.41, 95% CI 1.11 to 1.80; 45,619 men) (Taylor 2004); for supplements alone at that dose, a later update of the same cohort found 19% more (HR 1.19) (Ferraro 2016). Swedish men who took ascorbic acid tablets, typically 1,000 mg each, had about twice the risk of non-users (RR 1.92, 95% CI 1.33 to 2.77; 23,355 men) (Thomas 2013).

Women and food look different. In 156,735 women, neither total nor supplemental vitamin C was linked to stones, and dietary vitamin C wasn't linked to stones in men or women (Ferraro 2016). A likely mechanism: short controlled trials show 1-2 g a day raises urine oxalate, the main ingredient of most stones, on average though not in everyone (Traxer 2003; Farkouh 2026). Stone specialists advise stone formers, particularly men, to avoid 1,000 mg a day or more (Khusid 2020).

Most kidney stones are calcium oxalate, and part of the vitamin C you absorb is broken down to oxalate. A double-blind, placebo-controlled crossover put 12 healthy people and 12 calcium oxalate stone formers on a fixed diet and gave 1 g of vitamin C twice a day for 6 days. Urine oxalate rose from 28.5 to 34.7 mg/day in healthy people (P = 0.008) and from 30.5 to 41.0 mg/day in stone formers (P < 0.001), and calcium oxalate saturation rose in both groups (Traxer 2003).

A labelled-oxalate study in 48 people taking 1,000 mg twice daily with meals found the effect was uneven: 19 of 48, stone formers and non-formers alike, were responders whose urine oxalate rose by more than 10%. In them, vitamin C raised oxalate absorption from food by 31% and the body's own oxalate production by 39%, and their stone risk index rose from 0.76 to 1.10 (Massey 2005).

Pooling 9 intervention studies, vitamin C supplements raised 24-hour urine oxalate by 9.72 mg compared with baseline and 6.45 mg compared with placebo (both p < 0.001), in stone formers and non-formers, with no effect on urine citrate or calcium (Farkouh 2026). In 3,348 people from three US cohorts, those taking 1,000 mg a day or more passed 6.8 mg/day more oxalate than those under 90 mg, four times the 1.7 mg/day difference between the highest and lowest quarters of dietary oxalate (Taylor 2008).

The trials lasted 3-6 days and measured urine, not stones, and the cohort analysis was a one-time urine survey. In two trials, including one of 47 stone formers given 1-2 g a day for 3 days, urine pH did not change (Traxer 2003; Baxmann 2003), so it is not a reliable way to acidify urine; it lowered pH only in a small series of people whose urine was very alkaline (Noureldin 2017). The NIH notes that high intakes can raise urine oxalate and uric acid and may contribute to stones, especially in people with kidney disorders, but that stones are not generally considered a risk in healthy people (NIH ODS 2025).

There is no stone-prevention dose of vitamin C. Adults need 90 mg a day (men) or 75 mg (women), and people who smoke need 35 mg more; half a cup of raw red pepper has 95 mg and a medium orange 70 mg, so food easily covers it. The upper limit for adults is 2,000 mg a day from all sources (NIH ODS 2025), but stone risk in men rose well below it: a total intake of 500-999 mg a day (median about 700 mg) carried an HR of 1.29 and 1,000 mg or more an HR of 1.43 (Ferraro 2016).

For people who have had a stone, urology reviewers advise avoiding supplements of 1,000 mg a day or more, particularly men, and checking 24-hour urine oxalate in anyone who starts vitamin C (Khusid 2020). The European Association of Urology calls vitamin C's role controversial but advises calcium oxalate stone formers to avoid excessive intake (EAU Guidelines). Trial investigators suggested keeping it below 2 g a day (Traxer 2003). The same reviewers say women who form stones should be cautious too (Khusid 2020). The guideline's general measures matter more: 2.5-3 litres of fluid a day, a normal calcium intake of 1-1.2 g and no more than 4-5 g of salt (EAU Guidelines).

Two large cohorts found more stones in men taking high-dose vitamin C. In the US Health Professionals Follow-up Study, 1,473 of 45,619 men without past stones formed one over 477,700 person-years, and a total intake of 1,000 mg or more a day carried an RR of 1.41 against less than 90 mg (Taylor 2004). A later update of the same cohort, with 40,536 men, confirmed it: total intake of 500-999 mg a day had an HR of 1.29 (95% CI 1.04 to 1.60) and 1,000 mg or more an HR of 1.43 (1.15 to 1.79); supplements of 1,000 mg or more had an HR of 1.19 (1.01 to 1.40) (Ferraro 2016).

In the Cohort of Swedish Men, 907 men took ascorbic acid tablets only; 31 formed a stone, against 405 of 22,448 non-users, over 1998-2009 (RR 1.92, 95% CI 1.33 to 2.77). Risk rose with use: RR 1.66 under 7 tablets a week and 2.23 at 7 or more. Multivitamin users had no higher risk (RR 0.86), and the absolute difference was 147 first stones per 100,000 person-years (Thomas 2013). An earlier, 6-year analysis of 45,251 US men aged 40-75 found no association (age-adjusted RR 0.78 at 1,500 mg or more a day) (Curhan 1996), and a meta-analysis of 4 studies found higher risk in men taking 250-499 and 1,000-1,499 mg a day of supplements but not significantly at 500-999 mg or above 1,500 mg (Jiang 2019).

In 85,557 women followed for 14 years, 1,078 formed a stone, and vitamin C intake was not associated with risk (RR 1.06, 95% CI 0.69 to 1.64, at 1,500 mg a day or more against under 250 mg); the authors judged routine restriction unwarranted (Curhan 1999). In 156,735 women in the two Nurses' Health Studies, which include longer follow-up of that first cohort, neither total nor supplemental vitamin C was linked to stones (Ferraro 2016). The meta-analysis found no significant link in women at any dose (Jiang 2019).

Dietary vitamin C was not linked to stones in men or women, though few people ate more than 700 mg a day (Ferraro 2016). The Swedish authors warned their supplement finding should not be applied to dietary vitamin C (Thomas 2013). An umbrella review of 76 meta-analyses listed kidney stones among the few harms tied to vitamin C supplements, not to vitamin C intake in general (Xu 2022).

In 5,721 US adults, those eating the most vitamin C were less likely to report a past stone (OR 0.86, 95% CI 0.73 to 0.95) (Li 2025). A larger survey of 26,438 adults found no link with intake, and genetic analysis found no significant causal effect of blood vitamin C on stones (OR 1.66, 95% CI 0.80 to 3.46) (Jian 2021). Serum vitamin C was not linked to past stones in an earlier national survey either (Simon 1999).

These are one-time snapshots of diet and stone history, so they can't show which came first, and people who have had stones may change their diet. They mostly measure food rather than tablets. They don't outweigh cohorts that followed people forward and found the excess with supplements in men.

No randomised trial has tested vitamin C against stone events. The cohorts excluded people with past stones, so the figures describe first stones, not recurrences; the Swedish authors note that because stones often recur, the absolute increase is probably larger than the first-stone figure suggests (Thomas 2013). The only data in stone formers taking vitamin C long term come from a retrospective review of 24 patients with very alkaline urine, given a median of 1,000 mg a day to acidify it: urine pH fell from 7.6 to 6.9, 2 patients needed their dose cut or stopped for new high oxalate, and 8 (33.3%) formed new stones, mostly struvite and carbonate apatite, with no comparison group (Noureldin 2017).

This page's moderate evidence rating reflects consistent cohort and metabolic evidence about harm in men, not trials of prevention, which don't exist.

Stone risk is the main concern at high doses; the trial and cohort figures are in the evidence section above. Otherwise vitamin C has low toxicity: the most common problems at high doses are diarrhoea, nausea and stomach cramps, and the adult upper limit is 2,000 mg a day (400-1,800 mg in children, by age) (NIH ODS 2025). High intakes may contribute to stones especially in people with kidney disorders (NIH ODS 2025), so people with chronic kidney disease should agree any supplement with their kidney team. The NIH notes the best evidence that vitamin C contributes to stones is in people who already have high urine oxalate (NIH ODS 2025).

Oxalate can also damage the kidney directly. One case report describes acute kidney injury from oxalate deposits in a man taking 2 g of vitamin C a day after small bowel obstruction and resection, a setting that already raises oxalate absorption (Lin 2019). In hereditary haemochromatosis, high doses could worsen iron overload (NIH ODS 2025).

The stone-relevant interaction is with other products that add vitamin C or oxalate: many cranberry tablets contain vitamin C, and in a 14-day trial in 15 young adults urine oxalate rose with cranberry alone and much more with cranberry plus vitamin C; the authors advise people at risk of stones to avoid cranberry supplements (Redmond 2019). Beyond stones, the NIH lists two medicine interactions: antioxidants including vitamin C may interfere with some chemotherapy and radiation, and they may blunt the HDL rise from niacin with simvastatin (NIH ODS 2025).