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Multivitamins are one of the most commonly used supplements in the United States — around one-third of adults currently take them (1).

People who take multivitamins often do so to improve or maintain their health, protect themselves against nutrient deficiencies, or simply make up for the odd nutrient gap in their diet (1).

You can purchase multivitamins without a prescription, and most people view them as safe, which might explain their popularity. Despite this, multivitamins are not risk-free.

This article discusses whether you should be concerned about taking multivitamins. It reviews the potential side effects of multivitamins and which ingredients may cause them.

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Possible side effects of multivitamins

Your body needs to consume at least 13 vitamins and 16 minerals regularly to function properly.

A well-balanced diet is the best way to obtain these nutrients. That said, multivitamins provide a good alternative source for those who are unable to meet their nutrient requirements through diet alone.

Multivitamins that provide up to 100% of the daily Dietary Reference Intakes (DRIs) are generally considered safe and often free of side effects, as long as you take them as directed.

Nonetheless, some people may still experience a few side effects when taking multivitamins. Some side effects are more common than others (2, 3).

Common side effects

Certain side effects are more likely to occur than others when you’re taking multivitamins. These include (2, 3):

• constipation
• diarrhea
• upset stomach
• nausea

These gut-related side effects are generally minor and often temporary. They tend to disappear as your body gets used to taking the multivitamin.

Nevertheless, contact your healthcare provider for further assistance if your symptoms persist.

Rare side effects

Rarer side effects of multivitamins include (2, 3):

• headaches
• nosebleeds
• insomnia
• gout

These typically occur very infrequently, especially if the dosage of nutrients in your multivitamin does not exceed the daily safe upper limit (UL).

However, people who combine multivitamins with other supplements or eat significant amounts of fortified foods may exceed the UL for certain nutrients. This may increase their risk of side effects (4).

Food companies sometimes add nutrients to foods during the manufacturing process. These are fortified foods.

Some people may also experience severe allergic reactions to certain multivitamins, although this is very rare.

If you notice hives, difficulty breathing, or swelling of the face, tongue, lips, or throat after taking a multivitamin, seek emergency medical help immediately.

Side effects in infants and children

Side effects in children are similar to those that adults may experience. However, children are likely to experience them at much lower doses than adults.

In other words, children who take multivitamins may have a higher risk of consuming extremely high levels of nutrients, which can lead to nutrient overdoses and even death in severe cases.

Companies market many multivitamins specifically for infants and children. However, a recent study suggests that up to 65% of them contain nutrient levels above the safe upper limits (UL) for children (5).

This may explain why experts report that children who consume multivitamins have a high risk of exceeding the UL, particularly for vitamin A, folic acid, and zinc (4).

Giving your child an iron-containing multivitamin when they don’t need it may also cause them to overdose on iron. Scientists consider this to be a lead cause of poisoning in children 6 years old and under (4).

To reduce the risk of side effects or toxic overdoses, make sure to consult your healthcare provider before giving your child a multivitamin.

Summary

Multivitamins are generally safe as long as they provide nutrient levels that fall within the DRI guidelines. Some people experience gut-related side effects when they first start taking a multivitamin, but these usually resolve quickly. Other side effects are rare.

Dosage

The National Institutes of Health asserts that multivitamins providing nutrient levels that fall close to their DRI shouldn’t cause serious side effects. However, it’s important to note that the government doesn’t regulate multivitamins in the same way as it does medications (4).

This means there’s a risk that a multivitamin could contain higher levels of nutrients than its label states. Nutrient levels in some multivitamins may sometimes even reach or exceed the daily UL.

Others offer megadoses of certain nutrients that purposefully exceed the UL recommendations, capitalizing on some people’s belief that the more nutrients you ingest, the better the health effects.

Megadoses of some nutrients may not be particularly harmful, but extremely high doses of certain nutrients can seriously harm your health (4).

Nutrients to watch out for

Multivitamins generally contain three categories of nutrients:

• Water-soluble vitamins. These can dissolve in water and don’t usually accumulate in the body nor cause severe side effects if you take them in excess (e.g., B vitamins, vitamin C).
• Fat-soluble vitamins. These dissolve in fat and accumulate in the body, reaching toxic levels and potentially causing havoc if you take them in excess (e.g., vitamins A, D, E, and K).
• Minerals. These are inorganic elements that can accumulate in the body and sometimes cause harmful effects if you take them in excess (e.g., iron, iodine, zinc, copper, and selenium).

Some of these cause more side effects at higher dosages than others. The sections below highlight the nutrients from each category that may be particularly harmful if you take them at high doses.

Water-soluble vitamins

Excess intakes of water-soluble vitamins don’t typically cause severe side effects, even when intakes are close to the UL.

This is because your body tends to flush out excess intakes of these vitamins through your urine. Still, intakes that are several times over the UL may result in a variety of issues.

For instance, vitamin C intakes that are three times larger than the UL may cause cramps, diarrhea, nausea, vomiting, or migraines (6, 7).

Excess vitamin B3, also known as niacin, starting from intakes three times above the current UL may result in stomach pain, high blood pressure, vision problems, and liver damage (8).

Similarly, taking vitamin B6 at 10 times above the UL over the long term has been linked to skin lesions, light sensitivity, heartburn, and neurological problems (9).

Finally, excess intakes of vitamin B9, also known as folic acid, may weaken the immune system, cause neurological problems, and mask a severe vitamin B12 deficiency (10).

Fat-soluble vitamins

Multivitamins that offer large amounts of fat-soluble vitamins can be harmful, as excess levels of these vitamins can build up in the body.

For instance, excess intakes of vitamin A may cause headaches, liver damage, weaker bones, and birth defects (11).

Smokers and former smokers may especially benefit from avoiding multivitamins containing high amounts of vitamin A or beta carotene, which the body can convert into vitamin A. Getting too much of these nutrients may increase the risk of lung cancer (1, 4, 11).

Similarly, taking too much vitamin D, either due to manufacturing errors or taking an inappropriately high dosage, may result in nausea, vomiting, muscle weakness, cognitive problems, heart problems, kidney failure, and even death in severe cases (12).

Moreover, excess amounts of vitamin E may result in bleeding, diarrhea, weakness, blurred vision, and fertility problems (1, 13).

As for vitamin K, research has not found excessive intakes to cause too many problems. However, this vitamin can interact with various medications, including antibiotics and blood thinners.

People currently taking medications should let their healthcare provider know if they’re taking a multivitamin containing vitamin K (14).

Minerals

Like fat-soluble vitamins, minerals can accumulate in the body if you take them in excess, possibly causing harmful effects.

For instance, overly high intakes of certain minerals, such as iron, copper, magnesium, and zinc, can cause stomach upset, constipation, diarrhea, nausea, vomiting, stomach pain, and headaches (4, 15, 16, 17, 18).

Excess iron intake is particularly harmful because it may also cause liver problems.

For this reason, authorities advise adult men and postmenopausal women to avoid taking multivitamins that contain 18 mg of iron or more unless their healthcare provider advises them to do so (19).

Multivitamins containing high levels of iron, copper, and zinc may also prevent the body from absorbing other nutrients you consume (15, 16, 17).

Multivitamins containing too much iodine can cause thyroid problems. On the other hand, those with too much selenium may cause garlic-like breath, hair loss, brittle nails, or a metallic taste in the mouth (20, 21).

Selenium intakes above the UL may also cause severe neurological symptoms, kidney failure, and heart issues (21).

DRIs and ULs for each nutrient of concern

Most of the side effects mentioned in this article occur after a person consumes nutrient amounts that exceed the current upper levels (ULs).

Every nutrient has a UL at which scientists believe it becomes toxic. Exceeding a nutrient’s UL can lead to an overdose and severe side effects, such as liver damage and even death.

The following chart outlines both the DRI and UL for each nutrient of concern for adults.

Specific recommendations for infants and children vary widely based on their age. You can find more information in these exhaustive nutrient tables (22).

DRI for adult menDRI for adult womenULVitamin A900 mcg retinol activity equivalents (RAE)700 mcg RAE3,000 international units (IU)Vitamin B3 (niacin)16 mg niacin equivalents (NE)14 mg NE35 mgVitamin B6 (pyridoxine)1.3 mg1.3 mg100 mgVitamin B9 (folate)400 mcg dietary folate equivalents (DFE)400 mcg DFE1,000 mcgVitamin C 90 mg75 mg2,000 mgVitamin D 600 IU600 IU4,000 IUVitamin E 15 mg15 mg1,000 mgVitamin K120 mcg90 mcgNo UL establishedCopper900 mcg900 mcg10,000 mcgIodine150 mcg150 mcg1,100 mcgIron10 mg18 mg45 mgMagnesium420 mg320 mg350 mgSelenium55 mcg55 mcg400 mcgZinc11 mg8 mg40 mg

*Applies only to synthetic forms from supplements, fortified foods, or a
combination of the two.

It’s important to note that there’s no DRI available for folic acid, the synthetic form of vitamin B9 that you can find in multivitamins.

The form of vitamin B9 in the table above is called folate, and you can get it from natural sources — not multivitamins.

However, studies have found that dosages of the synthetic folic acid of more than 1,000 mcg per day may be associated with a range of negative health consequences in several populations (23, 24, 25).

Currently, researchers don’t fully understand the effects and safety of long-term intakes of vitamin or mineral dosages that fall between the DRI and UL. Therefore, scientists need to do more research on this topic.

Until more is known, it’s likely safest to avoid supplements that offer nutrient levels that exceed their DRIs.

Summary

Consuming multivitamins that contain nutrient levels exceeding the daily UL may result in an array of side effects. Scientists need to conduct more research to evaluate the effects of nutrient intakes that fall between the DRI and UL.

Other factors that may cause multivitamin side effects

Multivitamins may also become contaminated with harmful compounds, such as arsenic or lead (26, 27).

When you ingest these harmful compounds in large amounts or over a longer period of time, they may cause a variety of health issues, including physical, muscular, and neurological problems, as well as birth defects (26, 28).

It’s impossible to identify whether a multivitamin contains these harmful compounds by looking at its label alone.

However, some manufacturers opt to get their supplements verified by third-party labs, which can confirm whether they’re free of contaminants and that they truly contain what the label states.

Some examples of independent supplement testing companies include ConsumerLab, NSF International, and U.S. Pharmacopeia.

Summary

Multivitamins can become contaminated with harmful compounds, such as arsenic or lead. To minimize this risk, consider choosing multivitamins that a third-party lab has independently tested.

Short- versus long-term side effects

Depending on the nutrient and its dosage, some side effects may occur very quickly, while others may take a longer time to develop (11).

For instance, taking a very high dose of one or multiple nutrients may cause gut symptoms that generally develop shortly after taking the supplement (15, 16, 17).

However, more severe side effects may develop over time as excess amounts of nutrients or unwanted contaminants gradually accumulate in the body. These longer-term side effects may include birth defects and liver, heart, and cognitive issues (11, 20, 21, 27, 28).

If you think you’re experiencing any side effects, make sure to bring them up with your healthcare provider as promptly as possible.

Summary

The speed at which you may experience side effects depends on the type and dosage of nutrient you consumed. Make sure to discuss any side effects with your healthcare provider as soon as you notice them.

The bottom line

Multivitamins can be helpful for those who are unable to reach their daily nutrient needs through diet alone.

However, multivitamins cannot replace a balanced diet, and taking them in high amounts may result in side effects ranging from mild stomach upset to severe liver and heart problems.

Like many supplements, multivitamins are not strictly regulated and may contain much higher levels of nutrients than the label states. Depending on the nutrient, this will influence the speed and severity at which you may experience side effects.

You can minimize your risk by only taking multivitamins when you truly need them. Opt for ones that contain nutrient levels close to the current DRIs and have been tested by a third-party lab.