Multivitamin Side Effects, Including 8 Perks You May Notice When Taking One*

If there's one supplement stashed away in your medicine cabinet, chances are it's a multivitamin. Taken by a third of Americans1, multis make up a whopping 40% of all vitamin and mineral supplement sales in the U.S. Basically, they're pretty popular.

It makes sense: We have insurance for our cars and homes—so why not have an insurance policy for our diets and whole-body health, too?* Take a quality multivitamin regularly (we know, we know, it's easy to forget) and you can rest easier knowing you're providing your body with a baseline supply of the important vitamins, minerals, and other bioactives it needs to hum along smoothly.*

That said, not all of the multis out there are the same. In fact, according to mbg director of scientific affairs Ashley Jordan Ferira, Ph.D., RDN, the term multivitamin is "used loosely in the industry." Do a little perusing and you'll find everything from certain capsule brands and gummies that regularly offer less than 12 nutrients (which Ferira considers a complex, not a multi) to truly complete offerings with impressive formulas also under the same multi umbrella.

"A quality multivitamin should include a comprehensive array of essential water-soluble and fat-soluble vitamins, macrominerals, and microminerals at clinically relevant doses for daily nutritional support,"* she says. Better yet, a truly supreme formula might even "feature specialized botanical bioactives and phytonutrients to complement the nutrient array and incrementally support the customer's daily health, cellular functions, and longevity."*

As popular as multivitamins are, it's always a good idea to ask questions and be well informed about anything you put in your body. We're each unique—and there are a lot of different products out there—after all.

First and foremost, know this: "Multis are safe at their core," says Ferira. "There are always exceptions to the rule for irresponsible brands, but I'm talking about reputable, high-quality, clean brands here."

That said, there are a few things to be aware of when taking a multivitamin. One big one? In some cases, your multivitamin could interact with certain medications you take, according to dietitian Jessica Cording, R.D., CDN, author of The Little Book of Game-Changers. If you take medication to reduce blood clotting, for example, talk to your doctor before taking any supplement that contains vitamin K (or when significantly changing your dietary vitamin K intake, for that matter) since the vitamin contributes to blood clotting and can thus be counterproductive to blood thinning.* 

Research10 also suggests that smokers and former smokers may want to avoid large amounts of beta-carotene (the precursor to vitamin A found in plant foods and many supplements). Excess vitamin A11 may also have negative impacts on pregnant women, but as with most things in nutrition, there's some nuance to consider.

Ferira explains, "when we're talking about vitamin A toxicity, there are specific retinoic acid metabolites that are actually found in certain drugs, not supplements, that are genuinely bad for baby's development." Even though that type of vitamin A and science doesn't apply to supplements, "out of an abundance of caution, pregnant women should avoid supplements with 10,000 I.U. (3,000 mcg) or more of preformed vitamin A, which is the retinol form. This 'upper limit' doesn't apply to the beta-carotene vitamin A form at all."

One caution relevant to multivitamins that's super clear-cut is iron poisoning in young children. Ferira explains, "This concern of accidental pediatric iron overdose is exactly why multivitamins that contain iron include warnings on their labels, explicitly calling out this risk and directing the consumer to keep out of children's reach." So to repeat: Keep out of reach of children (who may mistake your multi capsule, tablet, etc., for candy).

Perhaps most notably, "multivitamins can promote sufficiency for a comprehensive array of essential vitamins and minerals in the body,"* says Ferira. In fact, research12 shows that supplementation with a multivitamin effectively helps to relieve micronutrient inadequacies—especially when taken consistently.*

Over time, consistently meeting your nutritional needs supports the body's ability to function optimally for the long haul. Don't think it could really amount to all that much? Data13 suggests that getting appropriate amounts of essential micronutrients is closely linked with longevity.* Also cool: Observational research14 has even linked multivitamin use with longer telomeres, which are considered a marker of biological age (and the longer, the better!).*

A large-scale scientific review17 shows that vitamin and mineral deficiencies (particularly related to vitamin A, vitamin B12, biotin, vitamin C, vitamin E, as well as iron and zinc) affect vision and eye health throughout the developed world. By promoting nutritional sufficiency, taking a multivitamin regularly may support the many functions in the body that contribute to eye health and vision.*

A number of vitamins and minerals that are shortchanged in many a multivitamin product, such as vitamin D, calcium, and magnesium, are all essential for proper bone health and muscle function.* Vitamin D, for example, helps the body to optimally absorb and maintain the proper balance of calcium, which is not only the major building block of bones19 but also a critical regulator and signaling molecule within muscle tissue.* Magnesium, meanwhile, is necessary in order for muscles to relax after contracting and has also been linked to bone health20.*

Considering all three of these have been identified as "nutrients of concern" in the U.S., meaning that a significant portion of the population doesn't get nearly enough of them, supporting sufficiency with a daily multi becomes an easy and important way to support our musculoskeletal and joint health.*

A variety of micronutrients—including vitamin A, vitamin C, vitamin E, biotin, silica, zinc, and copper—have been identified by researchers23 as having significant roles in promoting healthy skin.* Same goes for a number of unique antioxidant compounds, including the carotenoids lutein and lycopene. In fact, the authors of one large review published in Nutrients conclude that "The skin is a sensitive indicator of nutritional deficiencies. The most effective way to improve the condition of the skin is to supply it with essential nutrients."*

When it comes to our hair, in addition to high-quality protein from your diet, Ferira explains, "We also know that antioxidants, as well as sufficiency in B vitamin biotin24 and trace mineral silica25 play functional roles for the health of our hair, too."*

As for our nails, Ferira goes on to say that, "For an attentive health care practitioner, looking at the health of one's nails26 and how that changes over time gives major intel on micronutrient adequacy versus inadequacy."

Though not often talked about (and often left out of multivitamin formulas), vitamin K is an unsung hero of cardiovascular health because of its impacts on blood clotting and blood vessel function.* Of course, it's far from the only nutrient needed for healthy heart and cardiovascular function; a number of other vitamins and minerals—think multiple B vitamins plus minerals like calcium, potassium, and magnesium—also have major parts to play.* (Certain phytonutrients, like lutein27, zeaxanthin28, and lycopene29 also have clinical evidence demonstrating their ability to support heart health.)*

Making a (high-quality!) multivitamin/mineral supplement a regular part of your daily routine can help fill nutritional gaps in your diet and promote your overall health by supporting a variety of critical structures and functions in your body, including your immune system, eyes, brain, and more.* "The array of vitamins and minerals (and unique botanical ingredients, if included) all play critical roles in cells, tissues, and organs,"* Ferira says.