It is estimated that one-third of all U.S. adults and a quarter of children and adolescents take a multivitamin, making them some of the most popular supplements. Part of their popularity can be attributed to how easy and convenient they are at filling in any nutritional gaps in your diet.

While multivitamins, especially ones that come in fun flavors or gummy varieties, can seem pretty harmless, there can be consequences to getting too much of a good thing, says Kaylee Jacks, a sports dietitian at Texas Health Sports Medicine. We spoke with her to get her insight into the world of supplements and what risks are associated with them.

How Multivitamins Work

Multivitamins are formulated to provide at least the daily recommended dose of a variety of different essential vitamins and minerals. However, the specific nutrients and amounts of each provided per serving vary from product to product and brand to brand. They commonly include:

• Fat-soluble vitamins: A, D, E, and K
• Water-soluble vitamins: Vitamin C and the B-vitamins (thiamin, riboflavin, niacin, pantothenic acid, B6, biotin, folic acid, and B12)
• Minerals: zinc, iron, magnesium, calcium, selenium

“While multivitamins are a great idea, our bodies can only absorb and use so much of a specific nutrient at one time,” Jacks explains. “Because of this, absorption will decrease when intake is high.”

Beyond maybe wasting your money, Jacks notes that in most cases these excess vitamins and minerals don’t pose any threat because your body will filter and excrete whatever it doesn’t need. However, the same is not true for fat-soluble vitamins, whichare stored by your body, whether it needs them or not.

“This puts you at a greater risk for possible toxicity,” she adds. 

Side Effects to Watch Out For

Before we jump into toxicity, let’s discuss some common side effects associated with multivitamin use. Again, while most multivitamins are considered safe and typically side-effect-free when taken at the appropriate dose, some people are more susceptible to some uncomfortable symptoms.

Some common side effects include:

• Upset stomach: One of the most common side effects of multivitamin use is an upset stomach. This can include symptoms such as nausea, diarrhea, and constipation.
• Unpleasant taste: Some multivitamins can leave an unusual or unpleasant taste in your mouth.
• Tooth staining: Minerals, especially when taken in large doses, can cause side effects such as tooth staining.
• Increased urination: Minerals, especially when taken in large doses, can cause increased urination.

Reading labels and having a good understanding of how much you should be consuming at a maximum for each nutrient can go a long way in not only preventing some of the more unpleasant symptoms listed above but also reducing your risk of toxicity.

“It is important to make sure you are not consuming more than the Upper Limit (UL) for any given micronutrient to avoid possible toxicity,” Jacks explains. “ULs are typically much higher than the Recommended Daily Allowance (RDA) that’s listed on the bottle. That makes it a bit more unlikely that you’ll reach these levels, but it’s always good to be informed of what that limit is.”

For example, the RDA for vitamin C is 75 milligrams and 90 milligrams a day for women and men, respectively. This is a pretty typical dose you’ll find in one serving of most multivitamins. However, the UL for adults when it comes to vitamin C is 2 grams a day. Not including any vitamin C you’re getting from your diet, that means you’d have to consume more than 20 servings of a multivitamin to hit that limit. For reference, a glass of orange juice contains about 124 milligrams of vitamin C.

While reaching toxic levels of vitamin C can be a bit hard to achieve under normal circumstances, there are some vitamins and minerals you do need to be more mindful of.

For instance, the RDA for iron is 18 milligrams for women and 8 milligrams for men, while the UL is 45 milligrams. However, some popular multivitamins, especially those marketed to pre-menopausal women, can contain as much as 27 milligrams of iron per serving. That doesn’t include what you’re getting from your diet as well.

“It’s not uncommon for dosages in multivitamins to vary according to the age and sex they are marketed towards, so make sure to read the label carefully to get an appropriate supplement,” Jacks adds.

Again, Jacks reiterates that under normal circumstances, it’s rare to consume such a high amount of many vitamins and minerals to reach a toxic level, especially if they’re not fat-soluble, but it is still possible.

Some common vitamin toxicities that Jacks sees include:

• Vitamin D toxicity: This leads to calcium buildup, which can cause an increased risk for cardiovascular disease. This can also be caused by excess calcium supplements themselves, although high levels of calcium from the diet do not seem to pose the same risks.
• Vitamin A toxicity: This can lead to nausea, vomiting, and neurological problems.
• Vitamin E toxicity: Vitamin E decreases the availability of vitamin K, which is important for blood clotting.
• B6 toxicity: This can cause peripheral neuropathy, or weakness, numbness and pain from nerve damage.
• Iron toxicity: Iron increases free radicals in the body and leads to abdominal pain, vomiting, and diarrhea.
• Vitamin C toxicity: This can lead to abdominal pain and kidney stones.

If you experience any of these symptoms, as well as severe headache or insomnia, Jacks suggests stopping the use of the multivitamin and speaking to your doctor.

Who Should Avoid Multivitamins and Common Interactions

As we noted earlier, some people are more sensitive to vitamin toxicity than others, and this is especially true if you take certain medications, have certain health conditions, or are pregnant.

This can include:

• Smokers: Smokers should not consume high doses of vitamin A as it increases their risk for lung cancer.
• Pregnant women: Pregnant women should watch the amount of vitamin A they consume, as it can lead to an increased risk of birth defects. This can be a great time to reach for a prenatal vitamin over a multivitamin so you know you are getting the targeted support you need.
• People with certain medical conditions: People with impaired absorption due to conditions like Crohn’s disease, ulcerative colitis, celiac disease, or a history of gastric bypass surgery should speak to their doctor before taking multivitamins.

Additionally, there are many interactions between vitamins and medications, and caution should always be taken before starting a new medication or supplement.

Some common interactions include:

• Vitamin K: Vitamin K can decrease the effectiveness of medications used to thin the blood, such as warfarin. Because of this, you should consult your physician about any supplements you’re currently taking, thinking of taking, or considering stopping as Jacks notes that vitamin K intake or levels should stay consistent while these medications are being taken.
• Iron: Iron can decrease the effectiveness of certain antibiotics, thyroid medications, and medications for Parkinson’s disease (levodopa). Additionally, antacids reduce the absorption of iron and therefore should not be taken at the same time if you have been specifically prescribed an iron supplement to treat another condition, such as anemia.
• Calcium: Calcium can decrease the effectiveness of certain antibiotics. Corticosteroids can decrease the absorption of calcium while loop diuretics increase the excretion of calcium.

There are also many interactions between vitamins and minerals themselves. Some nutrients should not be taken together as they compete for absorption and can lessen the efficacy of each other, such as:

• Iron and calcium
• Vitamin C and vitamin B12
• Copper and zinc

On the other hand, there are some vitamins and minerals that play well with each other and should be taken at the same time to increase efficacy, such as:

• Vitamin C and iron
• Vitamin D and calcium
• Vitamin D and magnesium
• Fat-soluble vitamins and dietary fat

What to Be Mindful of When Considering a Multivitamin

While there are some side effects that can happen from taking multivitamins, and certain populations should be mindful of their intake, for the most part, Jacks reiterates that multivitamins are typically safe for the general public. However, she stresses that while it may be easier to reach for a supplement, your first approach should always be focused on consuming a well-balanced diet.

“Vitamin supplementation can be useful for some populations, such as older adults, vegans/vegetarians, and people living in climates where sun exposure is low,” she says. “But consuming a healthy diet, one made up of whole grains, fruits and vegetables, lean meats, and low-fat dairy, should provide all the necessary nutrients you need. Improving the diet should always be the first choice as the absorption of nutrients is better from food. Whole foods also contain other components which are important for your health, such as fiber.”

That being said, if you would still like to introduce a multivitamin into your diet, there are some things Jacks says you should be mindful of when shopping around.

“Because multivitamins have no regulatory definition as to what nutrients they must contain or in what amounts, they vary widely. You want to make sure third-party testing has occurred to ensure the adequacy of the product,” she explains. “NSF and USP are third-party organizations that test to make sure supplements contain the listed ingredients at the correct amount.”

Unfortunately, because supplements aren’t regulated, it’s not uncommon for many supplements to contain fillers and additives, some of which may be potentially harmful, such as:

• Titanium dioxide: a white colorant that causes free radicals and is classified as a carcinogen by the European Food Safety Authority (EFSA).
• Artificial colors/dyes
• BHT: a synthetic antioxidant
• Talc (Magnesium silicate): an anti-caking agent. Food-grade talc can be contaminated with low-grade talc that may contain asbestos.

It’s also becoming more common for many brands to include “special” ingredients such as herbs or adaptogens that are marketed to help your body respond to stress, anxiety, fatigue and or improve overall well-being. In addition to there being very little research to back up these claims, there are some people who should avoid these kinds of ingredients, such as pregnant women, those with certain health conditions, or people who are are taking certain medications that can interact with the ingredients.

The Takeaway

Multivitamins are generally safe and can give you the added comfort of making sure there are no gaps in your diet nutritionally. But Jacks says your focus should always start with changing up your diet first, then supplement later if necessary. If you experience any symptoms that you think may be associated with recent supplement use, you should stop the use and talk to your doctor. Additionally, if you are pregnant, plan to become pregnant, are nursing, have a health condition or are taking medications, you should speak with your doctor first before taking any new supplements.