Can You Overdose on Vitamins? A Dietitian Explains

Three-quarters of Americans make dietary supplements an indispensable part of their daily routine. Most of us want the best for our health, especially when we know our diet could use some work. However, as a registered dietitian, I ensure that my clients understand that just because vitamins are “over-the-counter” doesn’t mean we can disregard caution.

Are dietary vitamin supplements safe, no matter their dose? Short answer: no. As they say, the dose makes the poison. While this isn’t the case across the board, there are specific vitamins you want to be extra careful with.

Depending on your needs, dietary supplements can make a great addition to your health habits. And while some are safe to take in higher doses, others require a more mindful approach to avoid vitamin toxicity. Let’s talk about how you can make the best supplement choices for your individual needs.

What Is Vitamin Overdose?

While getting enough vitamins are imperative for our body’s function and overall health, more is not always better.

Vitamin overdose, also called hypervitaminosis, is when we consume well over the recommended amount of a certain vitamin, either acutely or chronically, resulting in negative health effects and potential toxicity.

• Acute: Taking a concentrated, high supplemental vitamin dose at one time that may cause immediate negative side effects.
• Chronic: Taking concentrated, high supplemental vitamin doses over a prolonged period of time that accumulates in the body that may eventually cause negative health effects.

Vitamin overdose often happens accidentally. 

There are many over-the-counter supplements that are considered megavitamins; supplements that contain higher-than-recommended levels of micronutrients. The amounts of micronutrients in these are often hundreds of times higher than our recommended daily intake. 

Because these are readily available, consumers may not consider them to be potentially risky. Yet taking these every day for years may cause vitamins to build up in the body and possibly give rise to long-term ill effects.

Additionally, we are bombarded on social media with health influencers telling us to take large doses of vitamins to cure what ails us. But just because “it worked for them” doesn’t mean that this information is right for everyone, and oftentimes goes against science, safety, and public health.

It’s always important to talk to your healthcare professional before adding a supplement into your daily routine.

Water-Solube vs. Fat-Soluble Vitamins

Each vitamin is metabolised and stored differently in the body. The main difference comes from whether they are water-soluble or fat-soluble.

Water-soluble vitamins (C and B-complex)

Water-soluble vitamins like vitamin C and B vitamins are not easily stored in the body and excesses are usually flushed out via urine. For this reason, regular intake is usually important to help maintain adequate levels so they can contribute to necessary biochemical functions in the body.

Even though these can’t be stored in the body, some of them can still cause side effects when taken acutely in high doses, or even if high doses are taken chronically over time. That’s why some of these vitamins have specific Tolerable Upper Limit Intake amounts (ULs), including vitamins C, B3, B6, and folate. Upper Limits (ULs) are the maximum daily dose that is unlikely to cause adverse health effects.

B vitamins include:

• B1 (thiamine)
• B2 (riboflavin)
• B3 (niacin)
• B5 (pantothenic acid)
• B6 (pyridoxine & five other active B6 compounds)
• B7 (biotin)
• B9 (Folate)
• B12 (cobalamin)

Fat-soluble vitamins (A, D, E, and K)

Fat soluble vitamins are absorbed and transported in a similar fashion to fats. They are also stored within our fat cells and the liver. This means that if we take in too much, our body does not get rid of it and excess will build up over time, resulting in a higher risk for toxicity.

So even though we need enough of these for optimal health, much more caution should be taken when consuming fat-soluble vitamin supplements.

Each of the fat-soluble vitamins, A, D, E, and K, have specific ULs to help prevent the adverse health effects that may develop after chronic, high-dose supplementation.

What Are the Symptoms of Taking Too Many Supplements?

Excessive intake of vitamins could result in immediate unpleasant side effects, such as gastrointestinal discomfort (known as acute toxicity), or more severe negative health consequences after prolonged supplementation (known as chronic hypervitaminosis).

Dietary Reference Intakes (DRIs) are reference values for each micronutrient, telling us how much of each vitamin and mineral healthy individuals need for optimal health. 

These were developed based on decades of research and collaboration between epidemiologists, physicians, chemists, and physiologists in the US and Canada, and include Recommended Dietary Allowances (RDAs) and Tolerable Upper Limits (ULs).

The signs of taking too many supplements can vary depending on the vitamin. Below, I’ll dive into the recommended doses and possible side effects of popular vitamins.

Vitamin C 

• Recommended Dietary Allowance (19+ years old)
  • Women: 75mg
  • Men: 90mg 
• UL: 2,000mg 

Side effects of acute high-dose vitamin C

While it is unlikely for vitamin C to become toxic at high levels, taking too much has been shown to potentially cause side effects such as diarrhea, nausea, gastrointestinal (GI) cramps, and other GI issues. This is because vitamin C is osmotic, meaning excess pulls extra fluids into the intestines.

Potential symptoms of vitamin C overdose

• May increase risk for kidney stones if the person already has hyperoxaluria
• May reduce copper and vitamin B12 levels, though more studies are needed
• May cause dental erosion, though more studies are needed

Vitamin B3 (Niacin)

• Recommended Dietary Allowance (19+ years old)
  • Women: 14 mg Niacin Equivalent (NE)*
  • Men: 16 mg NE
• UL: 35 mg (applies only to supplemental B3)

*The Food and Nutrition Board (FNB) defines 1 NE as 1 mg niacin or 60 mg of the amino acid tryptophan (which the body can convert to niacin).

Side effects of acute high-dose B3

Doses of 30 to 50mg of supplemental nicotinic acid may cause side effects such as flushing/reddening of the face, arms, and chest due to the dilation of blood vessels. This may also be accompanied by itchiness.

Potential symptoms of B3 overdose

Doses of supplemental nicotinic acid and supplemental nicotinimide higher than 1,000 to 3,000 mg can result in even more possible symptoms.

• Acute one-time high dose:
  • Gastrointestinal distress like nausea, heartburn, and abdominal pain
  • Very low blood pressure
  • Impaired glucose intolerance and insulin resistance
  • Blurry vision
• Chronic high-dose supplementation:
  • Nausea, vomiting
  • Liver toxicity or failure

Vitamin B6 

• Recommended Dietary Allowance
  • Men and women 19-50 years: 1.3 mg
• UL: 100 mg*

*It should be noted that in 2023 the European Food Safety Authority (EFSA) released an opinion recommending the UL be set at 12 mg

Side effects of high-dose B6

Acute supplementation of high dose B6 may not result in immediate side effects; however, chronic use of high-dose B6 supplements may have severe negative health consequences.

Potential symptoms of vitamin B6 overdose

• Nerve problems and loss of control of bodily movements
• Painful dermatological lesions
• Photosensitivity
• Gastrointestinal symptoms such as nausea and heartburn

Folate (B9)

• Recommended Dietary Allowance (19+ years old)
  • Men and women: 400 mcg Dietary Folate Equivalent (DFE)*
• UL: 1,000 mcg (applies only to supplemental folic acid, not folate from food)

*The FNB defines DFE as:

• 1 mcg DFE = 1 mcg food folate
• 1 mcg DFE = 0.6 mcg folic acid (from fortified foods or dietary supplements consumed with food)
• 1 mcg DFE = 0.5 mcg folic acid (on an empty stomach)

Side effects of high-dose B9

While acute high doses of supplemental folic acid may not cause immediate side effects, chronic high-dose supplementation has a few potential health effects.

Potential symptoms of folic acid overdose

• May accelerate progression of preneoplastic lesions
• High doses may exceed the body’s ability to break it down into tetrahydrofolate (THF). This excessive unmetabolized folic acid may impact the body’s immune system and may be linked with cognitive decline in older adults

Vitamin A

• Recommended Dietary Allowance (19-50 years old)
  • Men: 900 mcg Retinol Activity Equivalents (RAE)*
  • Women: 700 mcg RAE
• UL = 3,000 mcg

* The FNB defines RAE as:

• 1 International Unit (IU) retinol or 1 IU supplemental beta-carotene = 0.3 mcg RAE
• 1 IU dietary beta-caroteine = 0.05 mcg RAE
• 1 IU alpha-carotene or beta-cryptoxanthin = 0.025 mcg RAE

Side effects of acute high-dose vitamin A

Hypervitaminosis A, or acute vitamin A toxicity, can happen within a few days or weeks of taking exceedingly high doses of vitamin A. Side effects may include blurred vision, headache, nausea, dizziness, muscles aches, and coordination issues.

Potential symptoms of vitamin A overdose

• Dry skin
• Muscle and joint pain
• Depression
• Fatigue
• Hair loss

Preformed vitamin A (retinol)

Chronic excessive intake of Preformed Vitamin A (from animal products and supplements) may lead to more serious health problems, including birth defects in pregnant women. Taking high dose vitamin A or beta-carotene while smoking can increase these risk factors.

Beta-carotene

Chronic intake of high-dose beta-carotene supplements, or eating a very large amount of beta-carotene-containing plants, is more closely linked with carotenodermia, or the skin turning a yellow-orange hue. This can be reversed after stopping the high-dose supplement.

Vitamin D

• Recommended Dietary Allowance (19-70 years old)
  • Men and Women: 15 mcg or 600 IU
• UL: 100 mcg or 4,000 IU

Side effects of acute high-dose vitamin D

Side effects from high vitamin D intake in acute one-time doses are uncommon, but risks increase after chronic high-dose supplementation.

Potential symptoms of vitamin D overdose

• Hypercalcemia (high calcium levels), which may lead to: nausea, vomiting, muscle weakness, dehydration, and kidney stones
• Calcification of soft tissues in the body, including blood vessels
• Renal failure
• Cardiac arrhythmias

Vitamin E

• Recommended Dietary Allowance (14+ years)
  • Men and Women: 15 mg
• UL: 1,000 mg

Side effects of high-dose vitamin E

Vitamin E does not have many side effects from taking high-doses, both acutely or chronically. Still, high doses may affect the body’s ability to form blood clots, which might increase risk for bleeding.

Vitamin K

• Recommended Dietary Allowance (19+ years old)
  • Men: 120 mcg
  • Women: 90 mcg
• UL: There is no UL set for vitamin K since it has low potential for toxicity

Side effects of high-dose vitamin K

Vitamin K has had no negative health effects reported from acute or chronic high-dose intake. With that said, vitamin K plays an important role in our body’s ability to form blood clots. So people who are on certain medications should speak with their primary healthcare professional before beginning to take vitamin K supplements.

Who Is at Risk for Vitamin Toxicity?

Vitamin toxicity could happen to anyone who is taking in excessive amounts of vitamin supplements without the proper supervision and care of their healthcare professional. 

With that said, there are specific groups of people who may be even more susceptible, including:

• Children
• Pregnant and lactating women
• Older adults
• Individuals with liver or impaired kidney function
• Individuals taking multiple supplements and/or eating several fortified foods
• Individuals on specific medications

Remember that when taking in the recommended amounts of supplemental vitamins, side effects and toxicity are unlikely to happen. Taking vitamin supplements in the appropriate dosage is generally safe when used correctly for most people.

Always check in with your healthcare professional to ensure new supplements are the right fit for your health and lifestyle.

Can You Overdose on Greens Powder?

Greens powders are considered dietary supplements, as they feature a variety of nutrients and superfoods. Any supplement you take should be done so with discretion; consuming only the recommended dose as directed by the manufacturer.

This is particularly true of greens powders that include added vitamins and minerals. Some greens powders add micronutrients in much higher amounts, so it’s important to keep an eye on the Supplement Facts Panel to know exactly what you’re consuming.

Don’t forget to take into account the other supplements or fortified foods you may be consuming. All of these together may add up to a bigger vitamin dose than you’re expecting.

Greens Powder Brands to Watch Out for

The below greens supplements include added vitamins and minerals that exceed 100% of recommended daily values:

• AG1 Next Gen: Vitamins over 100% include C, E, B1, B2, B6, B12, and biotin
• IM8 Daily Essentials: Vitamins C, D, B1,B2, B3, B5, B6, B12, and biotin
• Huel Daily Greens: Vitamins C, D, E, K, B3, B6, folate, B12
• Supergreen Tonik: B1, B2, B6, B12
• Biogena One: Vitamins C, D, B1, B3, B5, B6, folate, B12
• LTH Nourish Multi Plus Greens: Vitamins C, D, E, B1, B2, B12, biotin

Everyone’s health needs are unique, and the right greens powder for one person may not be right for others. Do your own nutrient research and consult a professional for the best advice.

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