Eleven vitamins, their published limits, and what excess does

Four of the eleven vitamins here have no published upper limit at all. Three have limits that count only what comes out of a bottle or a fortified cereal box, and two federal sources disagree about one of those three. One counts a single chemical form, whether it arrived in a capsule or a slice of beef liver.

That is not sloppiness. Each limit was built from whatever evidence of harm existed for that nutrient. It does mean any blanket sentence about upper limits is wrong about at least one vitamin here.

What follows is a reference. Every vitamin gets the same four things: what it does, the published figures, what deficiency causes and how often it happens here, and what excess does. These report what federal tables say. They are not recommendations, and nothing tells you to take, stop or change anything.

How to read these figures

Three labels recur in the table below, and all three describe populations rather than people: the Recommended Dietary Allowance or RDA is the target, the Adequate Intake or AI is what the board uses where an RDA could not be derived, and the Tolerable Upper Intake Level or UL is the ceiling. Vitamin K is the only vitamin here carrying an AI. Articles 322 and 323 explain how each is built and what a ceiling covers, so this page treats the figures as given.

Scope is what trips people. The National Academies print the default on the DRI tables themselves: “Unless otherwise specified, the UL represents total intake from food, water, and supplements.” The footnoted exceptions are where readers and writers go wrong.

Vitamin	Adult RDA or AI, 19+	Pregnancy	Adult UL, 19+	What the UL counts
A	900 mcg RAE men, 700 women	770 mcg RAE	3,000 mcg	Preformed A only, food and supplements
Thiamin (B1)	1.2 mg men, 1.1 mg women	1.4 mg	None set	n/a
Riboflavin (B2)	1.3 mg men, 1.1 mg women	1.4 mg	None set	n/a
Niacin (B3)	16 mg NE men, 14 mg NE women	18 mg NE	35 mg	Disputed; see below
B6	1.3 mg at 19-50; 1.7 men / 1.5 women at 51+	1.9 mg	100 mg	Food and supplements
Folate	400 mcg DFE	600 mcg DFE	1,000 mcg	Supplements, fortified food
B12	2.4 mcg	2.6 mcg	None set	n/a
C	90 mg men, 75 mg women	85 mg	2,000 mg	Food and supplements
D	15 mcg (600 IU) to 70; 20 mcg (800 IU) over 70	15 mcg	100 mcg (4,000 IU)	Not stated by ODS
E	15 mg alpha-tocopherol	15 mg	1,000 mg	Supplemental alpha-tocopherol
K	AI 120 mcg men, 90 mcg women	90 mcg	None set	n/a

All figures are the Food and Nutrition Board’s, as published on the NIH Office of Dietary Supplements health professional fact sheets. The adult limits apply unchanged in pregnancy and lactation; lactation RDAs run higher still for vitamins A, riboflavin, B6, B12, C and E.

Vitamin A, where the limit follows a chemical form

Vitamin A handles vision in dim light, immune function and cell growth. It arrives as preformed vitamin A, meaning retinol and its esters, from animal foods and supplements, and as provitamin A carotenoids, mostly beta-carotene, from plants. ODS states that the 3,000 mcg limit applies only to products from animal sources and to supplements whose vitamin A comes entirely from retinol or a retinyl ester, and that the board set no limit at all for the provitamin A carotenoids. The limit is form-specific and source-blind. In a multivitamin containing both, the carotene share is not what the limit is measuring.

ODS describes acute hypervitaminosis A within days to weeks of one or a few doses over a hundred times the RDA, bringing severe headache, blurred vision, nausea and coordination problems, with chronic excess producing dry skin, painful joints, fatigue and abnormal liver tests. Preformed vitamin A is also a teratogen, meaning it can cause birth defects: ODS reports that experts advise women who are or might be pregnant, and women who are lactating, against supplement doses above 3,000 mcg RAE (10,000 IU) daily. Beta-carotene is not known to be teratogenic.

Beta-carotene’s problem is not toxicity. Finland’s ATBC trial randomized 29,133 male smokers aged 50 to 69 and found lung cancer incidence 18 percent higher among those given beta-carotene, with total mortality 8 percent higher. CARET gave 18,314 American smokers, former smokers and asbestos-exposed workers beta-carotene plus retinyl palmitate, found a lung cancer relative risk of 1.28, and stopped 21 months early. US deficiency is rare: ODS reports fewer than 1 percent of Americans below a serum retinol of 20 mcg/dL. Globally, xerophthalmia, which begins as night blindness, remains a leading cause of childhood blindness.

The B vitamins: eight nutrients, three upper limits

The National Academies could not set limits for seven nutrients for lack of data, five of them B vitamins: thiamin, riboflavin, B12, pantothenic acid and biotin, plus vitamin K and carotenoids. No limit does not mean no ceiling, only that nobody found enough evidence of harm to draw one.

Thiamin and riboflavin

Thiamin, B1, runs energy metabolism and nerve function. Severe deficiency causes beriberi, but ODS says the more common US manifestation is Wernicke-Korsakoff syndrome, a neurological emergency, and reports that up to 80 percent of people with chronic alcoholism become thiamin deficient.

Riboflavin, B2, helps convert food into usable energy. ODS calls deficiency extremely rare here, with under 6 percent of the population below the Estimated Average Requirement, and says it has no known interactions with medications.

Niacin, where the flush starts at the limit

Niacin, B3, supports energy metabolism and DNA repair. Two authoritative sources describe its 35 mg limit differently. ODS says the limits apply only to supplemental niacin, and separately that no adverse effects have been reported from naturally occurring niacin in foods. The National Academies’ footnote says the limits for niacin, folate and vitamin E apply to synthetic forms from supplements, fortified foods, or both. A fortified cereal counts under one wording and not the other. Article 323 handles scope as a subject.

One number makes the UL concrete. ODS reports that 30 to 50 mg or more of nicotinic acid typically causes flushing, a red prickling heat from the widening of small blood vessels under the skin. The limit is 35 mg, so the limit and the first visible effect are the same number. Nicotinamide does not flush. At 1,000 to 3,000 mg a day ODS describes more serious effects, and says high doses over months or years can be hepatotoxic, more so with extended-release forms. Pellagra is very rare here.

Vitamin B6, and a limit built by halving

B6 supports over a hundred enzyme reactions, mostly in protein metabolism, and its 100 mg limit covers both food and supplements. ODS reports that 1 to 6 grams of oral pyridoxine daily for 12 to 40 months can cause severe, progressive sensory neuropathy, meaning nerve damage that disrupts sensation and coordination. Patients given an average of 200 mg a day for up to five years showed no such effect, so the board halved that figure. Frank B6 deficiency is relatively rare here.

Folate, and the thing it can hide

Folate builds DNA and red blood cells, and adequate folic acid around conception reduces the risk of neural tube defects, which is why US enriched grains have been fortified since the 1990s.

This is the clearest scope case here, and ODS carries no single sentence stating it. The 1,000 mcg limit sits in a table titled, in full, Tolerable Upper Intake Levels (ULs) for Folate from Supplements or Fortified Foods. A separate sentence says the board set no limit for folate from food, because high food intakes have not been reported to cause adverse effects. Together those give the scope: the limit covers synthetic folic acid, not the folate in spinach.

Then the trade-off. Fortification works, and it created a real concern. ODS explains that large amounts of folate can correct the megaloblastic anemia of B12 deficiency without correcting the neurological damage, and that some experts worry high folate intakes could mask a B12 deficiency until that damage is irreversible. The blood count improves while the problem continues.

Vitamin B12, and who genuinely cannot absorb it

B12 maintains nerves and red blood cells, and has no upper limit, which ODS attributes to its low toxicity. The absorption mechanism explains the at-risk list. B12 in food is bound to protein, must be freed by stomach acid, then handed to intrinsic factor, a carrier protein made by cells in the stomach lining. ODS reports absorption falls sharply once intrinsic factor’s capacity is exceeded, at 1 to 2 mcg, and runs about 2 percent at a 500 mcg dose and 1.3 percent at 1,000 mcg. A very large pill does not make a very large delivery.

The people who struggle are those whose stomachs cannot do that step, plus those not eating the vitamin. ODS names atrophic gastritis, which it says affects 2 percent of the general population but 8 to 9 percent of adults 65 and older; pernicious anemia; gastric and bariatric surgery; long-term metformin; proton pump inhibitors and H2 blockers; and people eating no animal foods. It puts deficiency at roughly 3.6 percent of adults 19 and older.

ODS states plainly that the neurological symptoms of B12 deficiency, such as numbness and tingling in the hands and feet, can occur without anemia. New unexplained numbness, tingling or balance trouble is a reason to see a clinician promptly, because that damage can become permanent.

Vitamin C, and the cold evidence people skip

Vitamin C builds collagen and assists iron absorption. ODS reports the board concluded people who smoke need 35 mg more per day, and is explicit that the 2,000 mg limit covers both food and supplements. Scurvy is rare in developed countries and fatal untreated; high intakes mostly cause diarrhea, nausea and cramps, since unabsorbed vitamin C draws water into the gut.

The cold evidence is weaker and more specific than the reputation. The 2013 Cochrane review by Harri Hemilä and Elizabeth Chalker pooled 29 trial comparisons covering 11,306 participants. Among the 10,708 in general-community trials, regular vitamin C gave a pooled risk ratio of 0.97 for catching a cold, confidence interval 0.94 to 1.00: no meaningful protection. Regular use did shorten colds, by 8 percent in adults and 14 percent in children.

Two findings cut against the common habit. In five trials covering 598 marathon runners, skiers and soldiers on subarctic exercises, regular vitamin C halved cold incidence, pooled risk ratio 0.48. That is a real effect in a narrow population under extreme physical stress, not in office workers in February. And in the therapeutic trials, where people started vitamin C after symptoms began, the review found no consistent effect on duration or severity. The thing most people do is the thing with the least evidence behind it.

Vitamin D, where two expert bodies disagree in public

Vitamin D regulates calcium absorption and bone mineralization and supports immune function. The ODS fact sheet does not state what its 4,000 IU limit counts, leaving the DRI default of total intake, a gap worth noticing in a nutrient this heavily supplemented.

The blood-level disagreement

ODS reports the board’s serum 25-hydroxyvitamin D figures. Below 30 nmol/L (12 ng/mL) is associated with deficiency and can cause rickets in children and osteomalacia in adults. From 30 to 50 nmol/L (12 to 20 ng/mL), some people are potentially at risk of inadequacy. Levels of 50 nmol/L (20 ng/mL) or more are sufficient for most people, and concentrations above 125 nmol/L (50 ng/mL) can be associated with adverse effects.

The Endocrine Society publishes no matching set. Its 2024 clinical practice guideline, led by Marie Demay in the Journal of Clinical Endocrinology and Metabolism, reports that no clear evidence defines the optimal target 25(OH)D level for disease prevention in the populations examined, and suggests against routine testing in them. Its 2011 predecessor had suggested serum measurement as the initial diagnostic test in at-risk patients. Two respected bodies differ on something basic, and this site does not resolve it. The 2024 guideline also says of itself that it was not designed to replace the Dietary Reference Intakes, the line most often dropped in summary. A 2011 paper by the board’s own committee, led by A. Catharine Ross, explains the restraint: higher values were not consistently tied to greater benefit.

What VITAL found

The largest relevant trial is VITAL, reported by JoAnn Manson and colleagues in the New England Journal of Medicine in 2019, it was a randomized, placebo-controlled trial with a two-by-two factorial design testing vitamin D3 at 2,000 IU a day and marine omega-3 fatty acids at 1 g a day. It enrolled 25,871 US participants, including 5,106 Black participants, and followed them a median of 5.3 years for invasive cancer and for a composite of heart attack, stroke or cardiovascular death.

The result, in the authors’ words: “Supplementation with vitamin D did not result in a lower incidence of invasive cancer or cardiovascular events than placebo.” Cancer occurred in 1,617 participants, hazard ratio 0.96; major cardiovascular events in 805, hazard ratio 0.97. A 2022 fracture analysis in the same cohort found no reduction in total, hip or nonvertebral fractures. One caveat matters: most participants were already sufficient, only 2.4 percent falling below 30 nmol/L at baseline. Article 326 covers what such trials do and do not settle.

Two ODS statements about skin color both hold, and quoting either alone misstates the evidence. Melanin reduces the skin’s production of vitamin D, and NHANES 2011 to 2014 showed 17.5 percent of non-Hispanic Black people at risk of deficiency against 2.1 percent of non-Hispanic White people; the same page says whether those lower levels carry health consequences is not clear, and that bone health in older Black American women does not appear to benefit from raising 25(OH)D above 50 nmol/L.

Toxicity is almost always a supplement story, ODS says, and runs through calcium: marked hypercalcemia above 11.1 mg/dL against a normal range of 8.4 to 10.2 mg/dL, with nausea, vomiting, weakness, excessive thirst and kidney stones. Article 733 covers sun exposure and names no dose or target level.

Vitamin E and vitamin K, the two that meet at bleeding

Vitamin E protects cell membranes. Read its scope carefully, because two nearby ODS sentences are not one claim. ODS says the limits apply to all forms of supplemental alpha-tocopherol. It says separately that no adverse effects have been reported from large amounts of vitamin E from food. The second is an observation about food, not a statement that the limit excludes it.

The harm evidence is direct. SELECT randomized 35,533 men to vitamin E at 400 IU a day as synthetic all-rac-alpha-tocopheryl acetate, selenium, both, or placebo. With extended follow-up, Eric Klein and colleagues reported in JAMA in 2011 that 620 men in the vitamin E group developed prostate cancer against 529 on placebo, a hazard ratio of 1.17 and an absolute increase of 1.6 cases per 1,000 person-years. ODS adds that trials have reported increased hemorrhagic stroke risk at high supplemental doses, a signal ATBC also produced. In 2022 the US Preventive Services Task Force graded vitamin E and beta-carotene D for preventing heart disease or cancer in nonpregnant adults, the only nutrients it recommends against.

Vitamin K runs blood clotting and bone metabolism, has no upper limit, and ODS says clinically significant adult deficiency is very rare, mostly limited to malabsorption disorders or interfering drugs. Then there is warfarin, the most clinically important interaction in this article. Warfarin and similar anticoagulants work by blocking vitamin K, which depletes the clotting factors that depend on it. ODS states that people taking these drugs need a consistent intake of vitamin K from food and supplements, because sudden changes can increase or decrease the drug’s effect. Note the word: consistency, not avoidance. Someone who quits salad after hearing kale is dangerous has changed their intake as surely as someone who doubles it, and the dose was set around the old habit. That belongs with the prescribing clinician; article 677 covers interactions in full.

What to check, and the one thing to carry away

Three questions of our own, from gaps that recur in the sources above. Which form is in the product, not just which vitamin, since retinol and beta-carotene share a label line and carry different limits. Whether a limit counts your food, since folate’s and vitamin E’s do not, and two federal sources disagree about niacin’s. And whether a figure you were handed is a population value or a personal result, since an RDA covers nearly everyone in a group, so most individual requirements sit below it.

Deficiency in healthy American adults is rare for most vitamins here; ODS calls it rare or very rare for vitamins A, B6, C, E and K, for riboflavin, and for pellagra. Documented harm from excess is specific, published, and in several cases came from randomized trials with tens of thousands of people. So hitting eleven targets is not the thing to audit. What matters is whether anything you take, across every product combined, pushes one nutrient toward a limit that exists because people were harmed above it. No label shows you that total, and a clinician or pharmacist who can see the whole list is the person to work it out with.

Sources

1. NIH Office of Dietary Supplements. “Vitamin A and Carotenoids – Health Professional Fact Sheet.” ods.od.nih.gov
2. NIH Office of Dietary Supplements. “Thiamin – Health Professional Fact Sheet.” ods.od.nih.gov
3. NIH Office of Dietary Supplements. “Riboflavin – Health Professional Fact Sheet.” ods.od.nih.gov
4. NIH Office of Dietary Supplements. “Niacin – Health Professional Fact Sheet.” ods.od.nih.gov
5. NIH Office of Dietary Supplements. “Vitamin B6 – Health Professional Fact Sheet.” ods.od.nih.gov
6. NIH Office of Dietary Supplements. “Folate – Health Professional Fact Sheet.” ods.od.nih.gov
7. NIH Office of Dietary Supplements. “Vitamin B12 – Health Professional Fact Sheet.” ods.od.nih.gov
8. NIH Office of Dietary Supplements. “Vitamin C – Health Professional Fact Sheet.” ods.od.nih.gov
9. NIH Office of Dietary Supplements. “Vitamin D – Health Professional Fact Sheet.” ods.od.nih.gov
10. NIH Office of Dietary Supplements. “Vitamin E – Health Professional Fact Sheet.” ods.od.nih.gov
11. NIH Office of Dietary Supplements. “Vitamin K – Health Professional Fact Sheet.” ods.od.nih.gov
12. National Academies of Sciences, Engineering, and Medicine, Food and Nutrition Board. “Dietary Reference Intakes (DRIs): Tolerable Upper Intake Levels, Vitamins and Elements (table footnotes).” nationalacademies.org
13. US Preventive Services Task Force. “Vitamin, Mineral, and Multivitamin Supplementation to Prevent Cardiovascular Disease and Cancer: Preventive Medication (final recommendation, June 21, 2022).” uspreventiveservicestaskforce.org
14. New England Journal of Medicine, via PubMed. “Manson JE et al. "Vitamin D Supplements and Prevention of Cancer and Cardiovascular Disease." N Engl J Med, 2019. DOI: 10.1056/NEJMoa1809944.” doi.org
15. Journal of Clinical Endocrinology and Metabolism (Endocrine Society clinical practice guideline), via PubMed. “Demay MB et al. "Vitamin D for the Prevention of Disease: An Endocrine Society Clinical Practice Guideline." J Clin Endocrinol Metab, 2024. DOI: 10.1210/clinem/dgae290.” doi.org
16. Journal of Clinical Endocrinology and Metabolism (Endocrine Society clinical practice guideline), via PubMed. “Holick MF et al. "Evaluation, treatment, and prevention of vitamin D deficiency: an Endocrine Society clinical practice guideline." J Clin Endocrinol Metab, 2011. DOI: 10.1210/jc.2011-0385.” doi.org
17. Journal of Clinical Endocrinology and Metabolism, via PubMed. “Ross AC et al. "The 2011 report on dietary reference intakes for calcium and vitamin D from the Institute of Medicine: what clinicians need to know." J Clin Endocrinol Metab, 2011. DOI: 10.1210/jc.2010-2704.” doi.org
18. Cochrane Database of Systematic Reviews, via PubMed. “Hemila H, Chalker E. "Vitamin C for preventing and treating the common cold." Cochrane Database Syst Rev, 2013. DOI: 10.1002/14651858.CD000980.pub4.” doi.org
19. New England Journal of Medicine, via PubMed. “Omenn GS et al. "Effects of a combination of beta carotene and vitamin A on lung cancer and cardiovascular disease." N Engl J Med, 1996. DOI: 10.1056/NEJM199605023341802.” doi.org
20. New England Journal of Medicine, via PubMed. “The Alpha-Tocopherol, Beta Carotene Cancer Prevention Study Group. "The effect of vitamin E and beta carotene on the incidence of lung cancer and other cancers in male smokers." N Engl J Med, 1994. DOI: 10.1056/NEJM199404143301501.” doi.org
21. JAMA, via PubMed. “Klein EA et al. "Vitamin E and the risk of prostate cancer: the Selenium and Vitamin E Cancer Prevention Trial (SELECT)." JAMA, 2011. DOI: 10.1001/jama.2011.1437.” doi.org