These 7 Supplements Can Be Toxic If You Take Too Much
- 5 days ago
- 9 min read
Key Points
Supplements can help fill nutritional gaps. But when taken in excess, some can be toxic.
Fat-soluble vitamins are most at risk of being toxic when taken at high levels.
Work with a registered dietitian to determine optimal supplementation doses for you.
If you're like a lot of Americans, you probably have a medicine cabinet full of vitamin and mineral supplements aimed at helping you meet your nutrient needs and boost your health. While supplements can help fill deficiencies in your diet, taking large doses of some of them—particularly over a long time—can be dangerous and even toxic.
Vitamins and minerals, also referred to as micronutrients, are essential in small quantities to sustain normal cellular and molecular functions in the body. While it's almost always recommended that you get your nutrients through food, the Academy of Nutrition and Dietetics acknowledges in a 2018 position statement that there are some instances when you may need supplemental support, such as during times of growth, pregnancy, lactation and aging.
Unless medically prescribed and supervised by a professional, though, taking megadoses of vitamins is not the answer. Let's examine which types of vitamins can be toxic, the symptoms of toxicity, and how to
safely take supplements.
Different Types of Micronutrients and How They're Absorbed
Most essential vitamins are water-soluble and are used when they are digested; they typically do not stay in your system for long periods, and are excreted in your urine if you ingest more than your body can use.
Vitamins A, D, E and K are fat-soluble; they need fat for absorption and can be stored in body tissues. Taking large doses of fat-soluble supplements is more likely to cause toxic effects. However, megadoses of certain water-soluble vitamins and minerals can also lead to hypervitaminosis (the medical term for vitamin toxicity).
Can Taking Large Amounts of Supplements Cause Toxicity?
Yes, taking large amounts of supplements can cause toxicity. That's why the Food and Nutrition Board of the National Academies of Sciences, Engineering and Medicine has set a Tolerable Upper Intake Level, or UL. This is the maximum daily intake unlikely to cause adverse health effects for vitamins and minerals; dosages above this can be harmful.
The side effects you could experience depend on factors including the supplement you are taking, dosage, duration, your health conditions and whether you take other vitamins, minerals or medicines. Side effects can range from mild symptoms like headaches to—in the most severe instances—death.
Here are the vitamins and minerals you should be careful not to take too much of. It's wise to work with a registered dietitian, who can help you determine the optimal dose for you.
Vitamin A
Vitamin A is a fat-soluble nutrient that plays a role in healthy vision, immune system function, reproduction and growth. Optimal intake is important in maintaining liver and heart health, too. Certain animal-based foods—like fish, organ meats, dairy products and eggs—contain the active form of vitamin A, also known as retinol (preformed vitamin A). Plant-based foods have beta carotene, or provitamin A, which can be converted to vitamin A in the body.
The Recommended Dietary Allowance (RDA) for vitamin A is listed in retinol activity equivalents (RAE) and varies based on age and sex. The RDA is 900 micrograms RAE a day for adult men and 700 mcg for adult women, per the National Institutes of Health. One egg provides 80 mcg RAE of vitamin A, according to the USDA.
Certain populations may need to supplement with vitamin A, such as those with cystic fibrosis, anemia and certain gastrointestinal diseases. The UL for preformed vitamin A includes intakes from all sources—food, beverages and supplements—and is set at 3,000 mcg for adults 19 and older. There are no established upper limits for beta carotene or other sources of provitamin A.
Too much preformed vitamin A can cause headache, blurred vision, nausea, dizziness, muscle aches and problems with coordination. In severe cases, it can lead to coma and death. When used during pregnancy, too much vitamin A may cause birth defects.
According to the NIH, high intake of beta carotene does not tend to cause the same problems, although too much might make your skin turn yellow (which will resolve once consumption is reduced). However, people who smoke should be cautious, as high doses of beta carotene have been associated with an increased risk for lung cancer and death.
Vitamin D
Vitamin D is an important fat-soluble vitamin that is involved in functions including building and maintaining healthy bones and teeth, immune system function, reducing inflammation and helping with glucose metabolism. Vitamin D deficiency has been linked to heart disease, diabetes, certain cancers, autoimmune diseases and neurological disorders, according to the National Library of Medicine's publication StatPearls.
Vitamin D is found in a limited number of foods, and is largely made in the body when your skin is exposed to sunlight—that's why it's called the "sunshine vitamin." Foods that contain vitamin D include fortified foods (like cereal, dairy products and plant-based milks), wild-caught fish like salmon, pasture-raised eggs, beef liver, cod liver oil, and mushrooms grown under UV light.
The RDA for vitamin D is 600 IU—or 15 mcg—for people ages 14 to 70, and there are various reasons why it's commonly taken as a supplement. Vitamin D supplements may help prevent osteoporosis, promote immunity and counter muscle loss for those who have suboptimal dietary intake, or during cold months when sun exposure is limited.
Vitamin D deficiency is fairly common—about 35% of U.S. adults are deficient, says StatPearls. To find out whether you're in that category, request a blood test from your primary care practitioner. If you have a vitamin D deficiency, supplementation may be required. The amount you need will vary depending on how low your blood levels are. Sometimes, supplement doses are recommended above the UL of 4,000 IU—or 100 mcg—for short periods of time to correct a deficiency.
But because vitamin D is fat-soluble, excess consumption for long periods of time can cause vitamin toxicity. While rare, it can happen, and usually occurs from supplement use. The NIH reports that extremely high levels of vitamin D in the blood (150 ng/ml or 375 nmol/l) can cause:
Hypercalcemia (too much calcium in the blood)
Nausea
Vomiting
Muscle weakness
Confusion
Pain
Loss of appetite
Dehydration
Excessive urination and thirst
Kidney stones
In 2024, the Endocrine Society updated its vitamin D guidelines regarding testing and intake, but they can be confusing for the average person and many variables can affect vitamin D levels. This is why an individualized approach that is supervised by a medical professional is safest.
Vitamin E
Vitamin E is a powerful antioxidant that is important for immune, cellular and vascular health. It can be consumed through vegetable oils, wheat germ, nuts, seeds, green leafy vegetables and fortified foods like breakfast cereals and margarine.
The RDA for vitamin E is 15 milligrams per day, according to the FDA, while the UL for natural or synthetic supplements is 1,000 mg daily (1,500 IU a day for natural vitamin E supplements and 1,100 for synthetic), per the National Academy of Sciences. The natural form of vitamin E is called alpha-tocopherol, and the synthetic form is DL-alpha-tocopherol. Some supplements are sold as mixed tocopherols.
People with gastrointestinal disorders that inhibit fat absorption may need to supplement with vitamin E. But too much supplemental vitamin E can increase the risk of bleeding by reducing the blood's ability to clot, which can increase the risk of hemorrhage, according to the NIH. This is of particular concern for people who take blood-thinning medication like Coumadin and warfarin.
Vitamin K
According to the NIH, vitamin K is a fat-soluble vitamin involved in blood clotting and is an important component of bone health. It's found in green leafy vegetables and can be taken in supplemental form. Most people do not need a vitamin K supplement, because it is found abundantly in the diet and deficiencies are rare. However, certain populations are at greater risk for low levels of K, including those who have had bariatric surgery and people with cystic fibrosis, celiac disease, ulcerative colitis and short bowel syndrome.
The Daily Reference Intake (DRI) for vitamin K is established as an Adequate Intake. which is used when evidence is insufficient to establish an RDA; intake at this level is assumed to ensure nutritional adequacy. It's set as 120 mcg per day, per the FDA.
There's no set UL for vitamin K because there is limited evidence of toxicity. However, people who take certain medications, like blood thinners, should not supplement with vitamin K. In some cases, too much could trigger allergic reactions and the potential for liver damage.
Vitamin B6
Vitamin B6 is a water-soluble vitamin that is involved in many bodily processes, including protein synthesis and immune and cognitive functions. It is found in a variety of foods, including fish, beef, organ meats, fortified foods, starchy vegetables and fruit.
The RDA for vitamin B6 is 1.7 mg daily for adults, according to the FDA. The UL of 100 mg a day is much lower than dosages associated with adverse effects.
According to the NIH, severe nerve damage can occur when vitamin B6 is taken in large quantities for a year or more, leading to a loss of bodily movements, although it usually goes away once the supplement is discontinued. Other potential effects of long-term excessive intake include increasing your sensitivity to the sun as well as nausea and heartburn.
Iron
Iron is a mineral and an essential component of hemoglobin. It supports muscle metabolism and is necessary for physical and neurological development, cellular functioning and oxygen transport. Two types of iron are found in food, including heme (found in animal products like red meat) and non-heme (available in plant foods such as nuts, seeds, whole grains and legumes). Heme iron has a higher bioavailability, meaning that it's more readily absorbed, but your body will absorb non-heme iron better when it's eaten with foods containing vitamin C.
The RDA for iron is 18 mg for adults, per the FDA. Iron needs increase during pregnancy and lactation. People who may benefit from an iron supplement include those who have iron deficiency or iron-deficiency anemia, those who follow a strict vegan or vegetarian diet, those who are pregnant or nursing and those who have heavy periods.
The NIH notes that supplemental iron (25 mg) can reduce zinc absorption. This dosage is commonly found in supplements and may be suggested to treat a deficiency. But this should be under the supervision of a medical professional. High-dose supplementation can increase the risk of constipation, nausea, abdominal pain, diarrhea and vomiting. While these symptoms are not toxic, they are certainly unpleasant. Short-term intake of much higher doses (20 mg/kg) can lead to necrosis (death) of intestinal cells, especially when not taken with food.
Zinc
Zinc is required to activate certain immune cells and acts as a powerful antioxidant. It is also important in wound healing. Zinc deficiency is associated with impaired immune function and increased risk of infections in certain groups.
Foods that are rich in zinc include seafood, grass-fed beef, organic chicken (dark meat), raw pumpkin seeds, cashews and chickpeas. The RDA for zinc is 11 mg a day, per the FDA. According the NIH, the UL is 40 mg a day for people ages 19 to 50.
Taking more than 100 mg per day can lead to adverse effects, like an increased risk of prostate cancer, fever, chills and/or headaches. Long-term zinc supplementation can result in a copper deficiency.
How to Safely Take Supplements
Vitamins and minerals taken above the Tolerable Upper Intake for extended periods of time can be harmful and sometimes toxic. However, there are times when supplementation is necessary. Supervised supplementation can be used to meet nutrient requirements or to treat a deficiency. The Academy of Nutrition and Dietetics notes that supplementation may be necessary if you are:
restricting energy intake for weight loss
not consuming adequate amounts of food because of poor appetite or illness
regularly eliminating one or more food groups from your diet
consuming low amounts of micronutrient-rich foods despite adequate or excessive energy intake
Certain groups are more vulnerable to micronutrient deficiencies due to age, increased needs or absorption problems. These groups include older adults, those who are pregnant or breastfeeding, alcohol-dependent individuals, those who follow strict vegetarian or vegan diets and those with increased nutrient needs due to medications or disease.
If you fit into one of these groups, you may need supplementation. The best way to take supplements safely is to consult with a registered dietitian (RD) who can assess your nutrient intake and establish safe levels of supplemental dosages.
If you are unable to meet with an RD, work with your healthcare provider and stick to doses with no more than 100% of the RDA. Look for supplements with independent verification, which ensures that the supplement you are purchasing contains what it says it does and is not contaminated with other ingredients. Companies like NSF, USP and ConsumerLab are independent organizations that review the manufacturing process of products. This can include product testing and facility audits.
The Bottom Line
Supplements can be used to fill nutrient gaps or to correct micronutrient deficiencies. Certain populations are at greater risk of developing insufficiencies and deficiencies. However, high-dose supplements without medical supervision, especially fat-soluble vitamins, can be toxic. Before starting any type of supplement, you should discuss it with a medical professional, preferably a registered dietitian.
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