High-Potency and "Megadose" Multivitamins: What Label Amounts Mean

Urgent first

If a child may have swallowed a multivitamin, especially one containing iron, or a gummy or chewable vitamin, call Poison Help at 1-800-222-1222 (US) or 911 if they are unwell, and do not try to make them vomit. The NHS says very high doses of iron can be fatal, particularly in children. [1] See Multivitamin, iron and gummy overdose and Poisoning: what to do first.

What "high potency" and "megadose" mean

The terms are used on labels, but the sources we read do not define either one. They are not official categories. The useful question is simple: how much of each nutrient is in a serving, in which form, and how does that compare with official upper levels for your age group and country? Those comparisons below are reference points, never targets or advice. The amount that is right for a person is for a clinician or pharmacist to decide.

Official upper levels side by side

EFSA (EU) defines the tolerable upper intake level (UL) as the maximum total chronic daily intake from all dietary sources not expected to pose a risk of adverse effects in humans. [2] The NHS (UK, pages last reviewed 2020) gives amounts it says are unlikely to cause harm. Different bodies use different methods and dates, so figures differ. All are for adults; children's limits are lower, and EFSA lists them by age. [2]

Not every vitamin has an upper limit: EFSA lists none for vitamin K, and none could be derived for vitamin C. [2] "No upper limit" does not mean "proven safe at any amount". Vitamin D, iodine and selenium also have EFSA limits (100 micrograms, 600 micrograms and 255 micrograms a day for adults); see Vitamin D, Iodine and Selenium. [2]

What trials found with high amounts

• Beta-carotene in smokers. In a trial of 29,133 male smokers aged 50 to 69 in Finland, 20 mg a day of beta carotene for 5 to 8 years was linked to 18% more lung cancer than no beta carotene; 50 mg of alpha-tocopherol did not reduce lung cancer, and there were more deaths from hemorrhagic stroke in that group. [10]
• Beta-carotene plus retinol. In 18,314 smokers, former smokers and asbestos-exposed workers, 30 mg beta carotene plus 25,000 IU retinol a day raised lung cancer risk (relative risk 1.28), and the trial stopped early. [11]
• Official reviews. The US Preventive Services Task Force concluded the harms of beta carotene supplements outweigh benefits for preventing heart disease or cancer, found no net benefit of vitamin E, and found the evidence insufficient for multivitamins. [12]

Those amounts are study amounts in specific groups, not advice, and they do not show that lower amounts are harmful or that other nutrients behave the same way.

Doubling up

EFSA's limits count total intake. Some apply only to supplement and fortified-food forms, such as folic acid, or to preformed vitamin A. [2] A multivitamin, a separate B-complex, a fortified drink and a "hair and nails" product can each contribute the same nutrient. Add up the amounts per serving across everything, then ask a pharmacist. See How to evaluate a supplement formula.

Who should be most careful

Pregnancy and breastfeeding (vitamin A above; see Pregnancy and breastfeeding), children, older adults, and people with kidney or liver conditions or on several medicines should ask a clinician before any high-amount product. Water-soluble does not mean harmless: the B6 nerve damage above is one example. Product quality varies; see Quality testing and certifications.