Quick answer

For most healthy adults, yes. NIH says a basic multi near recommended amounts should be safe, and studies found only minor stomach complaints, which placebo takers had too. The risk is too much: in one study of 90,771 adults, 48 to 61% of multi users had excessive niacin intakes, and fortified foods and other supplements add to the total.^1,2

Key facts

Niacin in the nicotinic acid form typically causes skin flushing at 30 to 50 mg or more.³
NIH advises smokers to avoid multis with large amounts of beta-carotene or vitamin A.²
Biotin above recommended amounts can throw off some lab tests, including thyroid tests.⁴
Basic multis seldom interact with medicines; the main exception NIH names is vitamin K with warfarin.²

Taking one every day

Studies of multivitamin users report no adverse effects except minor stomach and gut complaints, which people taking a placebo had too.¹

NIH says a basic multi that supplies about the recommended amounts should be safe for healthy people. The concern it raises is less the multi on its own than everything taken alongside it.¹

Combined with fortified foods or other supplements, a multi can push some nutrients over their upper limits. NIH names iron, vitamin A, zinc, niacin and folic acid.²

High-potency products deserve a closer look, because some contain nutrients at or above an upper limit before any food or other supplement is counted.¹

Side effects of particular nutrients

Niacin in the nicotinic acid form typically causes flushing at 30 to 50 mg or more: the face, arms and chest turn red, with burning, tingling or itching. It can start within 30 minutes and is unpleasant rather than toxic, but it can come with headache, rash, dizziness or a drop in blood pressure. Taking it with food lessens it.³

Labels do not always say which form of niacin they use. If you flush after a multi, the label's niacin line is the first thing to check, and taking the multi with a meal is the first thing to try.³

Iron is the other usual suspect. High doses of iron can cause stomach upset, nausea and constipation, and NIH says taking iron supplements with food can help keep that to a minimum.⁵

Riboflavin, one of the B vitamins, is yellow. The body stores only small amounts, and any excess is either not absorbed or leaves the body in urine.⁶

Who should avoid one or ask first

In two trials, smokers who took 20 mg of beta-carotene a day, or 30 mg with vitamin A, had more lung cancer than those on placebo. NIH says smokers, and possibly former smokers, should avoid multis with large amounts of beta-carotene or vitamin A.¹

Too much preformed vitamin A (retinol) during pregnancy can cause birth defects. Beta-carotene does not carry this risk, and prenatal multis generally leave retinol out.^1,2,7

Accidental swallowing of iron supplements caused about a third of the poisoning deaths in US children reported from 1983 to 1991. Keep any product that contains iron out of children's reach.⁵

Smokers and former smokers: choose a multi without large amounts of beta-carotene or vitamin A²
Pregnant, could become pregnant or nursing: ask your doctor which multi to take
Children: use a children's product, and keep adult products that contain iron out of reach^2,5
Anyone taking prescription medicines: ask a pharmacist before starting

Lab tests and medicines

Biotin above recommended amounts can cause falsely high or falsely low results on some lab tests, including thyroid hormone tests. The levothyroxine label tells people to stop biotin supplements before thyroid testing.^4,8

Ask your pharmacist or doctor first

Vitamin K counters warfarin, and the warfarin label lists more vitamin K in the diet among the factors that weaken its effect. NIH says people who take warfarin should talk with their health care provider before taking any multivitamin that contains vitamin K.^1,9

On the medicine's label

Ask your pharmacist how far apart to take them

Calcium and iron can reduce how much levothyroxine is absorbed. The levothyroxine label tells patients that iron and calcium supplements can decrease its absorption, and MedlinePlus names calcium carbonate and iron supplements, and also niacin, among products that may interact with it. Many multivitamins contain calcium, and some contain iron.^5,8,10,11

On the medicine's label

Some antibiotics and methotrexate are also affected by the minerals or folic acid in multis. The medicine pages for multivitamins set out what NIH and the drug labels say about each.^12,13

Is long-term use safe?

The longest trials gave one multi every day for a median of 3.6 and 11.2 years, and NIH's summary of studies in multi users lists only minor stomach complaints. What changes over the years is usually everything else: a new fortified cereal, a vitamin D tablet or a protein powder can quietly raise the totals.^1,2

Looking again at everything you take once a year, with the upper limits from the dosage guide beside it, is a simple way to catch that drift.

Questions people ask

Is a multivitamin safe in the first trimester?

Ask your doctor, who will usually suggest a prenatal multi. Too much preformed vitamin A (retinol) in pregnancy can cause birth defects, which is why prenatal multis generally leave it out, and women who could become pregnant should already be getting 400 mcg of folic acid a day.^1,2,13

Why does my urine look bright yellow?

Riboflavin, one of the B vitamins in multis, is yellow, and the body passes the part it does not store in urine.⁶

Can a multivitamin upset your stomach?

It can. Minor stomach complaints are the one problem studies report, though people on placebo had them too. Taking it with a meal can help, especially if it contains iron.^1,5

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Sources

This page is information, not medical advice. Talk to your doctor or pharmacist about your own situation.