Multivitamin Side Effects: Timespan and When to Be Concerned - Healthline

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.

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).

*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.