Vitamin C Dosage Guide: How Much to Take (Why 1,000 mg Is Overkill)
Not medical advice — this summarizes published research; if you form kidney stones or take iron or other medication, discuss high doses with a clinician. Methodology.
Vitamin C absorption saturates around 200 mg — below that a dose is absorbed almost completely, but above it the fraction absorbed falls and the kidneys clear the rest. At a 1,250 mg dose, less than half is absorbed (PMID 8623000). So the RDA (75–90 mg) plus a personal ceiling of ~200–400 mg/day in split doses captures essentially all the benefit — past that your plasma is already saturated, so a 1,000 mg capsule barely raises your levels and the surplus is cleared in urine.
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Quick answer: aim for the RDA (75–90 mg) from food, and if you supplement, ~200 mg once or twice a day is the practical maximum worth taking — beyond that your blood level barely budges. The two situations where the number changes: take ~100 mg with a plant-iron meal if you're boosting iron absorption, and know that sustained doses of 1,000 mg+ carry a real kidney-stone signal in men. Use the estimator below to see how much of your dose you actually absorb.
How much of your vitamin C do you actually absorb?
Enter a single dose. Using the human pharmacokinetic curve (Levine et al.), we estimate how much is absorbed versus excreted — and whether splitting would help. (Estimates above ~1,250 mg are extrapolated beyond the doses actually tested.)
Why more vitamin C stops working: the saturation curve
The reason megadosing fails isn't opinion — it's some of the cleanest human pharmacokinetic data in all of nutrition. In tightly controlled inpatient studies, Mark Levine's NIH team depleted healthy volunteers of vitamin C, then gave carefully measured doses from 30 to 2,500 mg and tracked exactly what happened (PMID 8623000 in men, PMID 11504949 in women).
The result is a sigmoid (S-shaped) curve. Absorption is essentially complete up to about a 200 mg single dose, and plasma levels rise only marginally beyond an intake of roughly 200–400 mg/day (women saturate fully around 400 mg). Push past that and two things happen: the fraction absorbed drops (below 50% at 1,250 mg), and your kidneys excrete the surplus to hold blood levels steady. You physically cannot force your vitamin C level much higher by swallowing more — the body defends a ceiling. That's the whole case against the 1,000 mg habit in one sentence.
Vitamin C dose by goal
| Goal | Dose | Notes & evidence |
|---|---|---|
| General adequacy | 75–90 mg/day | The RDA; a diet with fruit/veg often covers it (NIH ODS). |
| Practical max worth taking | ~200–400 mg/day, split | Absorption is ~complete to ~200 mg/dose; plasma plateaus in this range (8623000, 11504949). |
| Iron absorption boost | ~100 mg with the meal | Converts plant (nonheme) iron to a more absorbable form and blocks inhibitors (6940487, 2507689) — strong for a single meal, smaller across a habitual mixed diet (11124756). |
| Cold — prevention | No proven benefit (general population) | Regular supplements don't cut how often you get colds (RR 0.97) (23440782). |
| Cold — under extreme physical stress | ~200 mg+/day regularly | Marathoners/soldiers in cold: ~52% lower cold incidence — the one prevention exception (23440782). |
| Smokers | RDA + 35 mg/day | Higher oxidative turnover raises requirement (NIH ODS). |
Does vitamin C actually help with colds?
This is the use everyone assumes works, and the honest answer is "partly, and not the way people think." The large Cochrane review (PMID 23440782) is clear on three points:
- Prevention: in the general population, taking vitamin C regularly does not reduce how often you catch colds (relative risk 0.97 across ~11,000 people). Routine use "isn't justified" for that purpose.
- Duration: regular supplementation does modestly shorten colds you get — about 8% in adults, 14% in children.
- Treatment after you're sick: starting high-dose vitamin C once symptoms have begun shows no consistent benefit — the classic "megadose at the first sneeze" move isn't supported.
The striking exception: people under extreme physical stress — marathon runners, skiers, soldiers in subarctic conditions — cut their cold incidence roughly in half with regular vitamin C. If that's not you, the cold case for daily vitamin C is weak.
Liposomal vitamin C: does it beat regular?
Liposomal products are marketed as "2–5× more absorbed," and there's a grain of truth under a lot of hype. Small trials do show liposomal encapsulation raises short-term blood levels above equivalent doses of plain ascorbic acid (PMID 39237620). But a 2025 scoping review of ten studies found the crucial gaps: none measured urinary excretion, so we can't tell whether those higher peaks are truly retained or just briefly higher before being excreted, and the few studies that looked at actual biological effects found minimal difference (PMID 40506693). One earlier study found higher blood levels but no improvement in the outcome it measured (PMID 27375360). Verdict: modestly higher blood levels, unproven real-world benefit — not worth a big premium for most people.
High doses and kidney stones
Sustained high-dose vitamin C carries a real kidney-stone signal — in men. Vitamin C is metabolized to oxalate, the main component of the most common kidney stones. In the Health Professionals Follow-up Study (45,619 men), those taking ≥1,000 mg/day had about a 41% higher stone risk versus lower intakes (PMID 15579526), and a Swedish cohort found ascorbic-acid supplements roughly doubled first-stone risk (PMID 23381591). The effect is male-predominant — large women's cohorts generally haven't shown it. If you form oxalate stones, avoid sustained high doses. Separately, doses near the 2,000 mg upper limit commonly cause diarrhea and stomach upset.
Forms & timing
- Ascorbic acid — the standard, cheapest, best-studied form (it's what the Levine studies used). Can irritate the stomach at large single doses.
- Buffered mineral ascorbates (sodium/calcium ascorbate) — the same ascorbate, pH-neutralized, gentler on the stomach; no better absorbed, just easier to tolerate.
- Ester-C — marketed as gentler/better absorbed; independent human data showing a meaningful edge is sparse.
- Liposomal — modestly higher short-term blood levels, unproven real-world benefit (see above).
- Split your dose. Because absorption saturates around 200 mg, two ~200 mg doses deliver more total absorbed vitamin C than one 400 mg hit (PMID 8623000).
- For iron: take ~100 mg with the plant-iron meal — timing drives the single-meal boost (PMID 6940487, 2507689); the effect is smaller across a habitual mixed diet (11124756).
Frequently asked questions
How much vitamin C should I take per day?
The RDA is 75 mg (women) / 90 mg (men), plus 35 mg for smokers. Because absorption saturates around 200 mg per dose and plasma plateaus at ~200–400 mg/day intake, there's little point taking more than ~200–400 mg/day, split into two doses. Diet often covers the RDA alone.
Does 1,000 mg do anything a 200 mg dose doesn't?
Not much. Absorption is nearly complete to ~200 mg, then drops sharply — below 50% at 1,250 mg — and the excess is excreted. A 1,000 mg megadose raises blood levels only marginally over 200 mg.
Will vitamin C stop me catching a cold?
No — regular supplementation doesn't reduce how often you catch colds in the general population. It modestly shortens colds you get (~8% adults, 14% children). The exception is people under extreme physical stress, who cut cold incidence ~50%.
Does taking vitamin C with iron help?
Yes — one of its most useful roles. About 100 mg taken in the same meal as plant (nonheme) iron makes the iron more absorbable and blocks inhibitors. Take it with the iron meal, not separately.
Can high-dose vitamin C cause kidney stones?
In men taking ≥1,000 mg/day long-term, yes — cohort studies found ~40–100% higher stone risk, via oxalate. Not consistently shown in women. Oxalate-stone formers should avoid sustained high doses; near-2,000 mg doses also cause diarrhea.
Is liposomal vitamin C worth it?
It raises short-term blood levels a bit more than plain ascorbic acid, but no study has confirmed better long-term retention or a real health benefit, and none measured excretion. The "2–5× absorbed" marketing outruns the data.
Related guides
- Vitamin C forms compared — ascorbic acid vs buffered vs liposomal · Best vitamin C supplement
- Vitamin C for immune system & colds · Iron dosage guide (pair them for absorption)
Sources
- NIH Office of Dietary Supplements. Vitamin C Fact Sheet for Health Professionals. ods.od.nih.gov
- Levine M, Conry-Cantilena C, Wang Y, et al. Vitamin C pharmacokinetics in healthy volunteers: evidence for a recommended dietary allowance. Proc Natl Acad Sci USA. 1996. PMID: 8623000
- Levine M, Wang Y, Padayatty SJ, Morrow J. A new recommended dietary allowance of vitamin C for healthy young women. Proc Natl Acad Sci USA. 2001. PMID: 11504949
- Hemilä H, Chalker E. Vitamin C for preventing and treating the common cold. Cochrane Database Syst Rev. 2013. PMID: 23440782
- Hemilä H, Chalker E. Vitamin C reduces the severity of common colds: a meta-analysis. BMC Public Health. 2023. PMID: 38082300
- Cook JD, Reddy MB. Effect of ascorbic acid intake on nonheme-iron absorption from a complete diet. Am J Clin Nutr. 2001. PMID: 11124756
- Lynch SR, Cook JD. Interaction of vitamin C and iron. Ann N Y Acad Sci. 1980. PMID: 6940487
- Hallberg L, Brune M, Rossander L. The role of vitamin C in iron absorption. Int J Vitam Nutr Res Suppl. 1989. PMID: 2507689
- Taylor EN, Stampfer MJ, Curhan GC. Dietary factors and the risk of incident kidney stones in men. J Am Soc Nephrol. 2004. PMID: 15579526
- Thomas LD, Elinder CG, Tiselius HG, et al. Ascorbic acid supplements and kidney stone incidence among men. JAMA Intern Med. 2013. PMID: 23381591
- Curhan GC, Willett WC, Rimm EB, Stampfer MJ. A prospective study of the intake of vitamins C and B6 and the risk of kidney stones in men. J Urol. 1996. PMID: 8618271
- Purpura M, Jäger R, et al. Liposomal delivery enhances absorption of vitamin C. Eur J Nutr. 2024. PMID: 39237620
- Davis JL, Paris HL, Beals JW, et al. Liposomal-encapsulated ascorbic acid: influence on vitamin C bioavailability. Nutr Metab Insights. 2016. PMID: 27375360
- Carr AC. Do liposomal vitamin C formulations have improved bioavailability? A scoping review. Basic Clin Pharmacol Toxicol. 2025. PMID: 40506693