Multivitamins are among the most widely utilized dietary supplements globally, with approximately one-third of adults in various countries, including India, Australia, Japan, and Canada, incorporating them into their daily regimen. Many individuals take these supplements to enhance or maintain their overall health, guard against nutrient deficiencies, or simply to compensate for occasional gaps in their dietary intake. Available without a prescription, multivitamins are generally perceived as safe by the public, which likely contributes to their growing popularity. However, it is essential to note that multivitamins are not entirely devoid of risks.

This article aims to explore whether there should be caution exercised when considering multivitamin consumption. It will delve into potential side effects associated with multivitamins and identify specific ingredients that may be responsible for these adverse effects.

Potential Side Effects of Multivitamins

To maintain optimal health, your body requires a regular intake of at least 13 vitamins and 16 minerals. A balanced diet is the ideal way to source these essential nutrients; however, for individuals who struggle to meet their nutritional needs through food alone, multivitamins can serve as a valuable alternative. Multivitamins that provide up to 100% of the daily Dietary Reference Intakes (DRIs) are typically regarded as safe and are often devoid of side effects when taken according to the instructions provided. Despite this, some individuals may encounter certain side effects while using multivitamins, with some effects being more prevalent than others.

Common Side Effects

When consuming multivitamins, the following side effects are more frequently reported:

• constipation
• diarrhea
• upset stomach
• nausea

These gastrointestinal side effects are generally mild and transient, often subsiding as your body adjusts to the intake of the multivitamin. However, if symptoms persist, it is advisable to consult your healthcare provider for further guidance.

Rare Side Effects

Less common side effects associated with multivitamins include:

• headaches
• nosebleeds
• insomnia
• gout

Such occurrences are infrequent, especially when the nutrient dosages in your multivitamin remain below the daily safe upper limit (UL). However, those who combine multivitamins with other supplements or consume large quantities of fortified foods may inadvertently exceed the UL for specific nutrients, increasing the likelihood of experiencing side effects. Fortified foods are those to which nutrients have been added during manufacturing.

Although rare, some individuals may experience severe allergic reactions to certain multivitamins. If you develop hives, face swelling, or difficulty breathing after taking a multivitamin, seek emergency medical assistance right away.

Side Effects in Infants and Children

Side effects in children mirror those observed in adults, although children may experience them at considerably lower doses. This means that children taking multivitamins could be at a heightened risk of consuming excessively high nutrient levels, potentially leading to nutrient overdoses and, in severe situations, fatalities.

Numerous multivitamins are marketed specifically for infants and children; however, a recent study found that up to 65% of these products contain nutrient levels that surpass the safe upper limits (UL) for children. This finding may clarify why experts indicate that children consuming multivitamins are at a significant risk of exceeding the UL, particularly for nutrients like vitamin A, folic acid, and zinc. Administering an iron-containing multivitamin to your child without medical necessity could also result in iron overdose, which is a leading cause of poisoning in children aged 6 and under. To minimize the risk of adverse effects or toxic overdoses, it is crucial to consult your healthcare provider before introducing a multivitamin to your child's regimen.

Dosage

The National Institutes of Health notes that multivitamins with nutrient levels close to their Dietary Reference Intakes (DRI) are unlikely to lead to significant side effects. However, it is crucial to recognize that the regulation of multivitamins does not match that of pharmaceuticals. This regulatory gap implies that some multivitamins might contain nutrient levels that are higher than what is indicated on their labels. In fact, certain multivitamins may reach or surpass the daily Upper Limit (UL) for several nutrients. Moreover, some products provide megadoses of specific nutrients that intentionally exceed UL recommendations, appealing to the misconception that higher nutrient intake directly translates to better health outcomes.

Nutrients to Monitor

Multivitamins typically contain three main categories of nutrients:

• Water-soluble vitamins. These vitamins dissolve in water and usually do not accumulate in the body; hence, they generally do not cause severe side effects if consumed in excess (e.g., B vitamins, vitamin C).
• Fat-soluble vitamins. In contrast, these vitamins dissolve in fat and can accumulate in the body, potentially reaching toxic levels and causing serious issues if taken in excess (e.g., vitamins A, D, E, and K).
• Minerals. These inorganic elements can also accumulate in the body, leading to harmful effects if consumed in excessive amounts (e.g., iron, iodine, zinc, copper, and selenium).

Some nutrients result in more severe side effects at higher dosages than others. The subsequent sections outline specific nutrients from each category that may pose risks when taken in large amounts.

Water-Soluble Vitamins

Typically, consuming excess amounts of water-soluble vitamins does not lead to severe side effects, even when close to the UL. This is primarily because the body tends to excrete excess amounts of these vitamins through urine. Nevertheless, intakes significantly exceeding the UL may lead to various health issues. For example, consuming vitamin C at three times the UL may result in cramps, diarrhea, nausea, vomiting, or migraines. Similarly, excessive intake of vitamin B3, or niacin, starting at three times the UL may lead to stomach pain, elevated blood pressure, vision issues, and liver damage. Long-term consumption of vitamin B6 at ten times above the UL has been associated with skin lesions, light sensitivity, heartburn, and neurological issues. Additionally, elevated intakes of vitamin B9, or folic acid, may impair immune function, lead to neurological problems, and mask a severe deficiency in vitamin B12.

Fat-Soluble Vitamins

Multivitamins containing high levels of fat-soluble vitamins can be detrimental, as excess levels can accumulate in the body. For instance, elevated intakes of vitamin A may trigger headaches, liver damage, weakened bones, and birth defects. Smokers and former smokers should particularly avoid multivitamins with high concentrations of vitamin A or beta-carotene, as an excess of these nutrients may increase the risk of lung cancer. Additionally, consuming too much vitamin D—whether through manufacturing errors or inappropriate dosages—can lead to nausea, vomiting, muscle weakness, cognitive difficulties, heart complications, kidney failure, and even death in extreme cases. Excessive vitamin E may cause bleeding, diarrhea, weakness, blurred vision, and fertility issues. Regarding vitamin K, while research hasn’t indicated severe problems from high intakes, it can interact with various medications, including antibiotics and blood thinners. Individuals on medication should inform their healthcare providers if they are taking a multivitamin that includes vitamin K.

Minerals

Like fat-soluble vitamins, excessive mineral intake can accumulate in the body and lead to adverse health effects. High intakes of certain minerals like iron, copper, magnesium, and zinc can result in stomach upset, constipation, diarrhea, nausea, vomiting, abdominal pain, and headaches. Excess iron intake is particularly concerning, as it may lead to liver complications. For this reason, health authorities recommend that adult men and postmenopausal women avoid multivitamins containing 18 mg of iron or more unless recommended by their healthcare provider. Multivitamins high in iron, copper, and zinc can also hinder the absorption of other nutrients in the body. Those containing excessive iodine can lead to thyroid dysfunction, while high levels of selenium may result in garlic-like breath, hair loss, brittle nails, or a metallic taste in the mouth. Intakes of selenium above the UL can also manifest severe neurological symptoms, kidney failure, and heart issues.

DRIs and ULs for Nutrients of Concern

Most side effects discussed in this article occur when nutrient intake exceeds the established upper levels (ULs). Every nutrient has a UL determined by scientists, beyond which toxicity is believed to occur. Surpassing a nutrient’s UL can lead to overdose and severe consequences, including liver damage and even fatality.

The following table summarizes both the DRI and UL for each nutrient of concern for adults.

Note that specific recommendations for infants and children vary significantly according to their age. More information can be found here.

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

It is important to note that no DRI exists for folic acid, the synthetic variant of vitamin B9 found in multivitamins. The vitamin B9 form listed in the table is folate, which can be derived from natural sources—not multivitamins. However, research indicates that consuming over 1,000 mcg per day of synthetic folic acid may lead to various negative health impacts across different populations. At this point, the long-term effects and safety of vitamin or mineral dosages that exist between the DRI and UL are not fully understood by researchers. Thus, more studies are necessary. Until further information is available, it is wise to avoid supplements that provide nutrient concentrations exceeding their DRIs.

Additional Factors Contributing to Multivitamin Side Effects

One significant concern regarding multivitamins is the potential for contamination with hazardous substances, such as arsenic or lead. These toxic elements, when consumed in substantial quantities or over extended periods, can lead to numerous health complications, which may include physical ailments, muscular dysfunction, neurological disorders, and even congenital disabilities.

The label on a multivitamin bottle does not provide a reliable means to determine the presence of these dangerous compounds. To ensure safety and efficacy, some manufacturers choose to have their products evaluated by third-party laboratories. These independent assessments can verify that the supplements are devoid of contaminants and accurately represent their ingredients as stated on the packaging.

Examples of reputable independent testing organizations include ConsumerLab, NSF International, and U.S. Pharmacopeia. By opting for multivitamins that have undergone such rigorous third-party testing, consumers can significantly reduce their risk of exposure to harmful substances.

Short-Term vs. Long-Term Side Effects

The occurrence of side effects can vary significantly based on the nutrient and its quantity; some may present themselves almost immediately, while others could take a considerable amount of time to manifest. For example, consuming an excessively high dose of one or several nutrients often leads to gastrointestinal symptoms that typically arise shortly after the supplement is ingested. In contrast, more serious side effects can develop gradually as the body accumulates excess nutrients or harmful contaminants over time. These long-term effects may encompass issues such as birth defects, as well as complications related to liver, heart, and cognitive functions.

If you suspect that you are experiencing any adverse effects, it is crucial to communicate your concerns to your healthcare provider as soon as possible. The onset of side effects is influenced by both the type of nutrient and its dosage. Hence, it is important to have a prompt discussion with your healthcare provider regarding any side effects you may observe.

The Bottom Line

Multivitamins can serve as a beneficial aid for individuals who struggle to fulfill their daily nutrient requirements through their diet alone. Nonetheless, it is essential to remember that they are not a substitute for a well-rounded diet. Excessive intake of multivitamins may lead to adverse effects, which can range from mild digestive discomfort to more serious issues affecting the liver and heart.

Similar to numerous dietary supplements, multivitamins often lack strict regulatory oversight, meaning they might contain nutrient levels that significantly exceed what is indicated on the packaging. The nature of the nutrient in question will dictate how quickly and severely any side effects may manifest.

To reduce your risk of experiencing these complications, it is advisable to consume multivitamins only when absolutely necessary. Choose products that offer nutrient amounts closely aligned with the current Dietary Reference Intakes (DRIs) and that have undergone testing by independent laboratories.