pouring multivitamins into hand

If you start your day by taking a multivitamin, you’re not alone. According to the National Institutes of Health, about a third of Americans regularly take a multivitamin. Among adults, women are more likely than men to take one, and multivitamin use increases with age. Multivitamins have been available since the 1940s, and over the decades, more and more Americans have turned to these convenient little pills.

Multivitamins are technically multivitamin/multimineral (MVM) supplements because they contain a broad spectrum of micronutrients. MVMs usually contain close to 100% of your daily needs for most micronutrients. However, they may include other minerals or vitamins (such as water-soluble B vitamins and vitamin C) in significantly higher amounts. MVMs may also be specially formulated to meet the nutritional needs of certain groups of people, such as men, women, seniors, or those who are extremely physically active.

MVMs may be particularly helpful for certain individuals at higher risk of nutritional deficiencies, such as seniors and those with GI conditions that negatively affect the body’s ability to absorb food. But should everyone be taking them, and are they really a necessity for optimal health? Here’s a look at what exactly happens to your body when you take a multivitamin every day.

Multivitamins can help fill nutritional gaps in your diet

Our bodies need dozens of vitamins and minerals for good health, and it can feel like a daunting task to construct and follow an eating plan to ensure you get all of them through food. If you’re a picky eater, don’t eat certain types of food because of allergies or intolerances, follow a diet that omits particular food groups, or simply don’t eat as healthy as you know you should, you may not be getting all the micronutrients you need.

According to Oregon State University, many individuals don’t meet their daily requirements of key vitamins and minerals. More than 94% of Americans, for example, don’t consume enough vitamin D, while 100% don’t get enough potassium. Other often under-consumed vitamins and minerals include vitamin E, magnesium, calcium, and vitamin A.

Taking a multivitamin can certainly help ensure you’re getting enough of the micronutrients you need, but it’s still best to get as much of these from real food as possible. As the Mayo Clinic explained, "Supplements aren’t intended to replace food. They can’t replicate all of the nutrients and benefits of whole foods, such as fruits and vegetables." Food has the advantage because it contains fiber and protective compounds like antioxidants.

You may not actually be getting what’s listed on the label

Unfortunately, multivitamins, along with all dietary supplements, fall into a regulatory gray area. The Food and Drug Administration (FDA) doesn’t have the authority to review the safety or effectiveness of dietary supplements before they’re put on the market. It’s up to manufacturers to ensure that their products are safe and contain the quantity and quality of ingredients they claim to have. If serious, large-scale problems emerge once a supplement is in circulation, the FDA can take the product off the market if it’s found to be unsafe or if the manufacturer’s claims about the supplement are deemed false and misleading.

If you want to ensure the multivitamin you’re taking is on the up and up, look for bottles with the "USP Verified Mark." The United States Pharmacopeia (USP) program is an independent certification program that evaluates the quality of dietary supplements. If you see the label on a bottle, you can be assured that the product contains the listed ingredients in the amounts and potency claimed. This USP verification also indicates that a product doesn’t contain harmful levels of certain contaminants, such as mercury, lead, and pesticides. Additionally, it certifies that the supplement breaks down in the body in the way and at the speed it’s supposed to. Finally, the USP label denotes that the product was made according to the FDA’s Good Manufacturing Practices.

If your multivitamin is old, it may not be doing anything for your health

If you’re one of those people who has an on-again, off-again relationship with multivitamins, you’ve likely acquired a cabinet full of bottles of expired supplements. The good news? Multivitamins past their best-by date aren’t dangerous in the same way expired food is. "Instead of becoming unsafe to ingest, they simply become less potent. That’s because most of the ingredients in vitamins and dietary supplements break down gradually," according to Healthline. "This means that they become less effective over time." The shelf life of vitamins is about two years, although chewable and gummy vitamins (which absorb moisture more easily) will lose potency sooner. If a supplement looks or smells strange, that’s a clear sign it’s time to toss it.

Storing multivitamins correctly can help extend their shelf life. Unfortunately, the two most common places to store supplements, the kitchen and the bathroom, are probably the two worst options in your home. As physician Jennifer Ashton explained in an interview with CBS News, both rooms are plagued by heat and humidity, the two most destructive forces regarding vitamin shelf life. Storing in the refrigerator can also degrade quality and longevity. Instead, Ashton recommends storing vitamins in an airtight container in a dry place, away from direct sunlight and heat sources.

Your multivitamin may interfere with medications you’re taking

Most people assume that multivitamins are, at best, a way to safeguard and improve your health and, at worst, an unnecessary but otherwise harmless waste of money. But multivitamins, like all dietary supplements, may interact negatively with over-the-counter and prescription medications.

The U.S. Pharmacist outlined a number of these potential interactions. For example, retinoid medications such as Accutane (used to treat acne) and Soriatane (used to treat psoriasis) are chemically similar to vitamin A. When taken in conjunction with this micronutrient, it can lead to vitamin A toxicity. Additionally, vitamin E may interfere with the action of chemotherapy drugs, while vitamin K may decrease the effectiveness of the blood thinner warfarin.

Minerals can also interact with prescription and over-the-counter medications. Calcium, magnesium, and iron can prevent the absorption of several types of antibiotics; in the case of ciprofloxacin, calcium reduces bioavailability by as much as 40%. Iron can reduce the effectiveness of levodopa, a drug to manage Parkinson’s disease, by 51%. Potassium can interact negatively with ACE inhibitors used to treat high blood pressure. But drug-supplement interactions are a two-way street. Corticosteroids and diuretics can reduce your body’s ability to absorb and retain calcium. Omeprazole, a non-prescription drug used to treat acid reflux, can reduce iron absorption.

You might be creating an micronutrient tug-of-war inside your body

Your daily multivitamin contains dozens of vitamins and minerals in a single pill, but that doesn’t mean they’re all team players. Some micronutrients work synergistically with one another, but some may interfere with the absorption or action of others. For example, on its own, your body only absorbs about 30% of the calcium you consume. Vitamin D, however, can enhance absorption rates. But other interactions aren’t as friendly. Vitamin A can interfere with the absorption of vitamin K, while vitamin E may interfere with vitamin K’s ability to do its job of clotting blood (via San Francisco Chronicle).

According to HelpGuide, a Harvard publication, vitamin C improves the absorption of iron but can inhibit the absorption of copper. Vitamin E allows the body to absorb and store vitamin A. Getting too much phosphorous can inhibit your ability to absorb magnesium. Adversarial relationships are especially common among trace minerals, which include iron, copper, zinc, selenium, and manganese, among others. If, for example, your multivitamin contains too much manganese, it can hamper iron absorption. HelpGuide cautioned, "Generally, food is a safe source of trace minerals, but if you take supplements, it’s important to make sure you’re not exceeding safe levels."

A multivitamin can keep your red blood cells healthy

If you’ve been diagnosed with anemia, you should consider taking a multivitamin that contains iron. According to the Mayo Clinic, iron-deficiency anemia is a common condition, especially among women. There are four potential causes of iron deficiency. Some individuals, particularly vegetarians and vegans, may not get enough iron in their diet. Others may have difficulty absorbing iron in the intestines because of gastrointestinal surgery or a condition such as celiac disease. A temporary increase in iron needs, such as during pregnancy, can also lead to deficiency. Additionally, blood loss, such as from a heavy period or ulcer, can rapidly deplete iron stores. Symptoms include extreme fatigue, shortness of breath, and muscle weakness. For those at risk of having low iron, taking a multivitamin that contains iron may be a good choice.

The National Institutes of Health estimates that between 14% and 18% of Americans take a supplement that contains iron. Multivitamins geared toward women usually contain 18 milligrams (100% of the daily value) of iron, while those formulated for men and seniors usually have little to no iron. The forms of iron most often used in multis include iron salts such as ferrous sulfate, ferrous gluconate, ferric citrate, and ferric sulfate. Each contains a different percentage of elemental iron. It’s important to note that calcium inhibits iron absorption; if your multivitamin contains both micronutrients, you may not actually be getting the iron you think you are.

Head off muscle cramps with a daily multivitamin

If you regularly get painful muscle cramps or twitches, it may mean you aren’t getting enough potassium or magnesium, both of which are usually included in a multivitamin. The National Institutes of Health (NIH) recommends adult men get 3,400 milligrams of potassium daily, while non-pregnant women should aim for 2,600 milligrams.

Potassium is considered a "nutrient of public health concern" because many Americans don’t get enough. Low potassium levels can lead to sudden, painful muscle cramps. As Healthline explained, potassium helps relay signals from the brain to the muscles, causing them to contract. When potassium leaves muscle cells, this causes muscle contractions to end. But "when blood potassium levels are low, your brain cannot relay these signals as effectively. This results in more prolonged contractions, such as muscle cramps," the site explained.

Low magnesium can also cause cramps and twitches. As McGill University explained, "Deficiency in magnesium lowers the electrical threshold at which nerve cells become depolarized." In other words, nerve cells become hyper-excited, randomly firing off the electrical signals rather than working as part of a coordinated team. According to the NIH, adults should get between 310 milligrams and 420 milligrams of magnesium daily, depending on age, sex, and pregnancy status. But approximately 48% of Americans don’t consume the recommended amount of magnesium.

Protect your eyesight by taking a multivitamin

If you’re not a big fan of carrots but want to keep your eyes healthy, taking a multivitamin that contains vision-supporting micronutrients can help. Vitamin A is likely the first thing that comes to mind when you think of micronutrients that promote eye health, and there’s a good reason for that.

According to Healthline, vitamin A is needed to produce rhodopsin, a protein that allows the eyes to function in low light. Vitamin A is also important for maintaining a clear cornea (the outside covering of the eye) and may reduce your risk for cataracts. But there are many other vitamins that play a critical role in propping up your peepers. Vitamin E’s antioxidant powers, for instance, combat the damage from free radicals that are believed to be a cause of many eye conditions.

In addition to being an antioxidant, vitamin C is also needed to produce collagen, a protein that provides structure for tissues throughout your body, including the cornea and sclera (two outer layers of the eye). A number of B vitamins have been studied in relation to specific eye conditions. Vitamins B6, B9, and B12 lower circulating levels of the inflammatory protein homocysteine, which has been implicated in age-related macular degeneration. Niacin (vitamin B3) may help prevent glaucoma, while riboflavin (vitamin B2) could ward off cataracts.

A daily multivitamin may not strengthen your bones

If you’re hoping a multivitamin is all it takes to keep your skeleton sturdy, think again. While multivitamins may contain the nutrients critical to bone health, they likely don’t contain them in amounts large enough to be useful. Although many vitamins and minerals are important for building and maintaining strong bones, perhaps the two most critical are calcium and vitamin D. About 99% of the calcium in the body is stored in bone, and your body uses this as a sort of "bank" it can draw from when there isn’t enough calcium circulating in the blood. Over time, if more calcium is "withdrawn" than "deposited," bones become brittle and fracture-prone. Vitamin D assists with absorption of calcium in the intestines and prevents the kidneys from excreting calcium in the urine. This ensures your body metabolizes and hangs on to a larger percentage of the calcium you consume from food or supplements (via Oregon State University).

Unfortunately, multivitamins can’t offer 100% (or even close) of the calcium you need because of simple logistics. As UT Southwestern Medical Center explained, "some nutrients, such as calcium, can’t be included in a multivitamin at 100% – if it was, the multivitamin would be too large to swallow." When it comes to vitamin D, a report published in the Arhices of Diseases in Childhood found that only 25–36% of multivitamins sampled contained the recommended 400 IU of vitamin D.

Keep your immune system in fighting shape with a multivitamin

It takes a whole team of micronutrients to keep your immune system in tip-top shape so a multivitamin can be a great way to ensure you’re getting the variety of vitamins and minerals you need. As a 2020 paper published in Nutrients noted, "The complex, integrated immune system needs multiple specific micronutrients, including vitamins A, D, C, E, B6, and B12, folate, zinc, iron, copper, and selenium, which play vital, often synergistic roles at every stage of the immune response."

The proper functioning of the immune system may require more of these micronutrients than the recommended dietary allowance (RDA). This means that even if you eat a well-balanced diet, a multivitamin may still be necessary to get all the micronutrients your immune system needs. The authors explained that even marginal deficiency in some of these micronutrients can impair immunity.

According to research conducted by Oregon State University and reported on by Science Daily in 2020, "Older adults who took a daily multivitamin and mineral supplement with zinc and high amounts of vitamin C … experienced sickness for shorter periods and with less severe symptoms." Those taking the multivitamin experienced fewer than three days of symptoms, while those taking the placebo had to suffer through more than six.

A multivitamin isn’t a magic bullet against heart disease

According to the Centers for Disease Control and Prevention, heart disease is a big problem in the United States. It’s the leading cause of death among Americans, claiming approximately 655,000 lives each year. While we often think of heart disease as a single condition, it actually encompasses a wide variety of diseases and disorders, including coronary artery disease (CAD), heart arrhythmias, congenital heart defects, and heart infections. While some conditions have a genetic or structural cause, CAD, the most common form of heart disease, is largely the result of diet and lifestyle factors (via the Mayo Clinic).

Because the causes of heart disease are so complex, don’t assume simply taking a multivitamin is all that’s needed to keep your ticker in good shape. As physician Edgar R. Miller III explained in an interview with Johns Hopkins Medicine, "The bottom line is, we don’t recommend supplements to treat or to prevent cardiovascular disease. Supplements [are] ineffective and unnecessary." In fact, getting too much of certain micronutrients in a multivitamin may actually do more harm than good. Miller noted that overdoing it on calcium and vitamin D from supplements may actually increase your risk for heart disease.

If you have type 2 diabetes, take a careful look at what’s in your multivitamin

The American Diabetes Association (ADA) estimates that 34.2 million Americans (more than 10% of the population) have diabetes. Of these, 1.6 million have type 1 diabetes, and 32.6 million have type 2 diabetes. As the Centers for Disease Control and Prevention explained, "If you have diabetes, your body either doesn’t make enough insulin or can’t use the insulin it makes as well as it should." Most of the food we eat is eventually broken down into the simple sugar glucose. Insulin is a hormone produced by the pancreas that allows glucose to enter cells, where it can be used as fuel. If the pancreas doesn’t produce insulin (type 1 diabetes) or cells become resistant to the effects of insulin (type 2 diabetes), excess glucose can build up in the bloodstream, causing damage throughout the body over time.

If you have diabetes, it’s a good idea to take a close look at your multivitamin’s label, as several commonly included vitamins and minerals may impact your condition, particularly if you’re also taking diabetes medications. Niacin (vitamin B3), for instance, can raise fasting blood glucose levels among diabetics. On the other hand, people with type 2 diabetes taking the medication metformin have lower levels of vitamin B12, so making sure you’re getting this vitamin in your multi may be important (via ADA).

Taking a multivitamin can keep your nervous system happy, especially if you’re vegan

Among its many other biological roles, vitamin B12, also known as cobalamin, is essential for the proper development and function of your central nervous system. It’s also required to produce myelin, a protective sheath that covers nerve fibers.

Vitamin B12 is only found naturally in meat, dairy, and eggs, though it can be added to fortified plant-based foods, according to the National Institutes of Health Office of Dietary Supplements. So if you abstain from meat and animal products, consider selecting a multivitamin that contains B12 to ensure you’re getting enough. Vitamin B12 deficiency is very common among vegans. According to research published in the American Journal of Clinical Nutrition, as many as 92% of vegans (compared to 66% of vegetarians and only 5% of omnivores) are deficient in this nutrient (via WebMD).

Because of vitamin B12’s involvement with the nervous system, particularly myelin creation, deficiency can lead to nerve damage. As Healthline explained, long-term B12 deficiency can cause myelin to form differently, which prevents nerves from working properly. This can produce a prickling pins-and-needles sensation in the hands and feet called paresthesia.

You may be getting too much of a good thing if you take a multivitamin every day

If you’ve got a "the more the merrier" attitude toward multivitamins and other supplements, you may want to rethink your strategy. While a single multivitamin shouldn’t cause any issues, taking multiple multivitamins or combining a multivitamin with numerous other supplements could lead to you getting way more of certain micronutrients than your body needs.

Andrew Shao, senior vice president of scientific and regulatory affairs for the Council for Responsible Nutrition, told WebMD, "If you’re taking a basic multivitamin, there’s no need to fear taking too much. Most multivitamins have such a wide margin of safety that even when you’re combining them with fortified foods, it’s still not going to cause you to keel over." That said, three micronutrients to watch out for are calcium, vitamin D, and folate. Individuals may get too much of these because they’re everywhere — added to the food we eat, included in multivitamins, taken as individual supplements, and rolled into targeted products like "bone health boosters."

It’s also important to remember that the recommended dietary allowance (RDA) or adequate intake (AI) for some vitamins and minerals — the amount you need for good health — is surprisingly close to the upper tolerable limit (UL) — the highest amount you can take without risking your health. Vitamin A and iron are examples of micronutrients that have a UL that’s proportionately not that much larger than the RDA and can have serious health consequences if consumed in excess (via WebMD).