How much of a vitamin or mineral is too much?

Two numbers. The Recommended Dietary Allowance (RDA) is the daily intake that meets the needs of nearly all (97% to 98%) healthy people of a given age and sex²⁴. The Tolerable Upper Intake Level (UL) is the highest daily intake likely to pose no risk to almost everyone; it is a ceiling, not a target, and above it the risk of harm rises²⁴. Most ULs count everything you eat, drink and take, but where harm has come only from supplements or fortified foods, the UL counts only those²⁴. That is why the limits for vitamin E, niacin, folic acid and magnesium leave out what is naturally in food^3,5,6,13.

The numbers for adults. These are for people aged 19 and over who are not pregnant or breastfeeding. Pregnancy raises several of the recommended amounts and children's limits are much lower; the fact sheet cited in each row lists every age group.

Nutrient	Recommended a day	Upper limit a day	What the limit covers
Vitamin D1,26	15 mcg (600 IU); 20 mcg (800 IU) over 70	100 mcg (4,000 IU)	All sources
Vitamin A2	900 mcg RAE men; 700 mcg RAE women	3,000 mcg (10,000 IU)	Preformed vitamin A (retinol) only; beta-carotene does not count
Vitamin E3	15 mg	1,000 mg	Supplements only
Vitamin B64	1.3 mg; over 50, 1.7 mg men and 1.5 mg women	100 mg (Europe's food safety agency: 12 mg)	All sources
Niacin5	16 mg men; 14 mg women	35 mg	Supplements only (nicotinic acid or nicotinamide)
Folate6	400 mcg DFE	1,000 mcg of folic acid	Folic acid in supplements and fortified foods only
Vitamin C7	90 mg men; 75 mg women; 35 mg more if you smoke	2,000 mg	All sources
Iron8	8 mg men; 18 mg women 19 to 50; 8 mg women over 50	45 mg	All sources
Zinc9	11 mg men; 8 mg women	40 mg	All sources
Selenium10	55 mcg	400 mcg	All sources
Iodine11	150 mcg	1,100 mcg	All sources
Calcium12,26	1,000 mg; 1,200 mg for women over 50 and everyone over 70	2,500 mg to age 50; 2,000 mg after	All sources
Magnesium13	400 to 420 mg men; 310 to 320 mg women	350 mg	Supplements and medicines only

Signs of too much. These are the effects the fact sheets list. Many build slowly, so if one appears, tell your clinician about every supplement you take:

• Vitamin D: high blood calcium, with nausea, vomiting, muscle weakness, changes in mood or thinking, pain, loss of appetite, dehydration, heavy urination, thirst and kidney stones¹.
• Preformed vitamin A: dry skin, aching muscles and joints, tiredness, depression and abnormal liver tests; birth defects in pregnancy².
• Vitamin E: bleeding; two trials found more hemorrhagic strokes in people taking alpha-tocopherol³.
• Vitamin B6: tingling, burning or numbness, usually in the hands or feet, and loss of coordination; also painful skin patches, sensitivity to sunlight, nausea and heartburn^4,14.
• Niacin: flushing with burning, tingling and itching from 30 to 50 mg of nicotinic acid; at 1,000 to 3,000 mg a day, low blood pressure, raised blood sugar, blurred vision and, over months, liver damage⁵.
• Vitamin C: diarrhea, nausea and cramps⁷.
• Iron: upset stomach, constipation, nausea, vomiting and diarrhea; an overdose can kill a child⁸.
• Zinc: nausea, vomiting, loss of appetite and headaches; weeks at 50 mg or more lower copper and HDL cholesterol and weaken immunity⁹.
• Selenium: garlic breath and a metallic taste first, then hair loss and brittle nails¹⁰.
• Iodine: goiter, and an underactive or overactive thyroid¹¹.
• Calcium: constipation, gas and bloating; kidney stones¹².
• Magnesium from supplements: diarrhea, nausea and cramps; very large doses cause low blood pressure, an irregular heartbeat and, rarely, death¹³.

Adding it up. Nutrients turn up in more products than the one bottle you think of. Vitamin B6 is in B-complex, multivitamin, magnesium and zinc products and in some fortified drinks¹⁴. Calcium is in antacids such as Tums and Rolaids¹², magnesium in laxatives and antacids¹³, zinc in some denture creams and in nasal sprays sold for colds⁹, and iodine-only supplements often carry high doses, some above the upper limit¹¹. In US national survey data, supplement users were more likely to exceed the upper limit for folic acid and vitamins A, B6 and C²¹; among pregnant women, 33.4% went over the folic acid limit and 27.9% over the iron limit²². Add each nutrient across every product and fortified food before you compare it with the table. Our interaction checker flags supplements that compete with each other, and the supplement schedule shows how to space them.

Reading a label.

• Percent Daily Value (% DV) is the share of a single reference amount in one serving, there to compare products rather than to set a personal target^1,23. Some Daily Values sit above many people's RDA: the DV for vitamin D is 20 mcg (800 IU), above the 15 mcg (600 IU) RDA for adults up to 70¹, and the 18 mg of iron in a typical women's multivitamin is 100% DV but more than twice a man's RDA⁸.
• The mineral, not the compound. The Supplement Facts panel must give the weight of the mineral itself, such as the calcium rather than the calcium carbonate²³. When a product names only the compound, do the arithmetic: calcium carbonate is 40% calcium and calcium citrate 21%¹², and ferrous sulfate is 20% iron, so a 325 mg tablet of ferrous sulfate supplies about 65 mg of iron⁸. Calcium is absorbed best 500 mg or less at a time¹².
• Units. Labels must give vitamin D in micrograms, and 1 mcg equals 40 IU, so 4,000 IU is 100 mcg¹. For vitamin A, 1 IU of retinol is 0.3 mcg RAE, so 10,000 IU is 3,000 mcg². For vitamin E, 1 mg is 1.49 IU of the natural form (d-alpha-tocopherol) or 2.22 IU of the synthetic form (dl-alpha-tocopherol), so 400 IU of synthetic vitamin E is 180 mg³. Folate is listed in mcg DFE with any folic acid in parentheses; folic acid counts 1.7 times, so "680 mcg DFE (400 mcg folic acid)" means 400 mcg toward the 1,000 mcg folic acid limit⁶.
• Proprietary blends need to show only the blend's total weight, so you cannot tell how much of each herb or other ingredient you get; vitamins and minerals with a Daily Value must still be listed one by one²³. See supplements to skip.

Our vitamins and minerals picks leave out products above these limits without a reason, and our guide to supplement purity covers whether a label matches what is in the bottle. Not sure you need the supplement at all? Start with do you need a supplement.

How the limits are set

The Food and Nutrition Board of the National Academies sets each UL from the evidence on harm, and states that a UL "is not intended to be a recommended level of intake"²⁴. The deciding harm differs by nutrient: copper status for zinc⁹, hair and nail loss for selenium¹⁰, stomach and gut effects of iron salts for iron⁸, kidney stones with calcium supplements for calcium¹², and flushing for niacin⁵. For B6, the board took the average of 200 mg a day that patients had used for up to five years without neuropathy and halved it, giving 100 mg⁴. The committee that revised calcium and vitamin D in 2011 listed among its uncertainties "the need to set ULs based on limited data in order to ensure public health protection"²⁶. Agencies can disagree: Europe's food safety agency set the adult B6 limit at 12 mg a day in 2023⁴.

Vitamin B6 and nerve damage

Chronic doses of 1 to 6 g a day for 12 to 40 months can cause severe, progressive sensory neuropathy with loss of coordination; symptoms usually stop if the supplement is stopped as soon as they appear⁴. A 1983 case series described seven adults with ataxia and severe sensory loss after daily high-dose pyridoxine; four were severely disabled, and all improved after stopping¹⁵. Lower doses have been implicated too. Among 172 women with raised blood B6, 60% had neurological symptoms; the 103 with symptoms had taken a mean of 117 mg a day, much the same as a comparison group, but for longer (2.9 versus 1.6 years), and they recovered within six months of stopping¹⁶.

Australia's medicines regulator had received 250 reports of peripheral neuropathy linked to B6 products by 31 October 2025, and says the risk cannot be excluded at doses below 50 mg a day¹⁴. It has required a warning on products giving more than 10 mg a day since 2022, and from 1 June 2027 products with more than 50 mg a day will need a pharmacist's advice or a prescription¹⁴. It also flags a trap: the symptoms of too much B6 resemble those of too little, so people may take more to treat the problem it causes¹⁴. One case report describes a 73-year-old man with three years of worsening neuropathy and a blood B6 of 259.9 nmol/L whose only reported source was a multivitamin with 6 mg²⁵. A single case cannot show cause, but it fits the regulator's warning. More in our vitamin B6 article.

Preformed vitamin A

The vitamin A limit covers preformed vitamin A (retinol and retinyl esters) from food and supplements and is set to avoid liver damage, birth defects and toxicity in children². Beta-carotene is not known to cause birth defects; too much of it simply turns the skin yellow-orange². In a cohort of 22,748 pregnant women, more than 10,000 IU a day of supplemental vitamin A was linked to 4.8 times the rate of cranial-neural-crest birth defects seen with 5,000 IU or less (95% CI 2.2–10.5), with an apparent threshold near 10,000 IU a day¹⁷.

Iron

Adults with normal digestion rarely overload on food iron, but supplements of 45 mg a day or more can upset the stomach, 25 mg or more can reduce zinc absorption, and extreme doses can cause intestinal necrosis, organ failure and death⁸. In the US, iron tablets and capsules must carry the warning that accidental overdose of iron-containing products is a leading cause of fatal poisoning in children under 6, and child-resistant packaging has been required since 1978 for containers with 250 mg of iron or more⁸. People with hereditary hemochromatosis absorb too much iron; without treatment, they typically show iron damage by their 30s, including cirrhosis, liver cancer, heart disease and pancreatic damage⁸.

Zinc and copper

Zinc's limit is set by its effect on copper⁹. According to a few reports, overusing denture creams with up to 34 mg of zinc per gram can lead to sensory ataxia, myelopathy (spinal cord damage) and anemia⁹. In four patients with neurological disease, low copper and high zinc, every one had used very large amounts of denture cream over long periods, and copper supplements brought mild improvement in two who stopped¹⁸. A review of 70 patients prescribed zinc found that 62% received doses high enough to cause copper deficiency, copper was measured in only two, 9% developed unexplained anemia and 7% neurological symptoms typical of copper deficiency¹⁹.

Selenium

In 2008, a liquid supplement that held 200 times its labeled selenium caused 201 cases of poisoning in 10 states²⁰. Diarrhea (78%), fatigue (75%), hair loss (72%), joint pain (70%), nail changes (61%) and nausea (58%) were common, and 90 days later 52% still had fingernail problems and 29% hair loss²⁰. The investigators wrote that holding the maker to pharmaceutical manufacturing standards might have prevented it²⁰. Food can cause trouble too: Brazil nuts carry 68 to 91 micrograms of selenium per nut and could cause toxicity if eaten regularly¹⁰.

Vitamin D below the "toxic" range

Overt vitamin D toxicity is unlikely below 10,000 IU a day, but the Food and Nutrition Board noted that intakes under the UL might still cause harm over time and advised avoiding blood levels above about 125 to 150 nmol/L (50 to 60 ng/mL)¹. Reported toxicity has come almost entirely from supplements, through manufacturing errors, misuse or prescribing mistakes¹.

Totals, not single products

In NHANES 2003 to 2006 (8,860 adults), supplement users had higher total intakes of every vitamin studied and fewer intakes below requirements, but also more intakes above the UL for folic acid and vitamins A, B6 and C²¹. Among 1,003 pregnant women, 69.8% used supplements; 33.4% exceeded the UL for folic acid, 27.9% for iron, 7.1% for zinc and 3.0% for calcium, even as many fell short on vitamin D, magnesium and iron²². About 5% of adults aged 51 to 70 exceed the folic acid limit, mainly because of supplements⁶.

Bottom line (confidence 3 of 5)

Upper limits are cautious estimates built from case series, cohorts and expert review, not from trials designed to find a threshold, and agencies disagree on some, B6 most of all. The harms above them are nonetheless real, repeatedly documented and almost always caused by supplements. Meeting the RDA from food plus a targeted supplement keeps most people far below the limits; a dose above a UL belongs under a clinician's supervision.