More Isn’t Always Better: 7 Vitamins You Can Overdose On

by Alf Orpen May 1, 2025

Most people think they’re doing their body good by taking an extra vitamin. But there are vitamins you can overdose on.

“Too much of anything can be bad”, a toxicologist told me, “Drink too much water and you can drown.”

More isn't always better. In some cases, it can be downright dangerous.

The following 7 vitamins are the ones you want to avoid overdosing on.

7 Vitamins You Can Overdose On

1. Vitamin C

Some supplements contain more than 500mg ascorbic acid - Between 5 - 12 times the Recommended Daily Allowance (RDA), depending on what country you live in (1)

But your body can’t store extra vitamin C, so the excess converts to oxalates and you pee it out. (2) It’s wasteful, and can even be painful...

Oxalates bind to calcium in your body to make calcium oxalate crystals, i.e. kidney stones. (3) Yes, that's right, too much Vitamin C can lead to kidney stones!

Below is an example of the difference between a conventional synthetic Vitamin C composition and a Wholefood Vitamin composition is.

2. Iron

Your body is great at absorbing iron, but has no way of getting rid of excess.

If you take too much iron it can accumulate in your body over time, leading to high iron stores (ferritin). Chronically high ferritin levels are associated with inflammation, heart disease, and liver disease. (4)

Haemochromatosis is usually an inherited condition causing you to absorb more iron. If it's not treated, it can damage your liver, joints, pancreas and heart. (5)

Below is an example of the difference between a conventional synthetic Iron supplement composition and a Wholefood Iron composition is.

3. Iodine

The RDA for iodine is 150 μg. Taking more than 600 μg per day puts you at risk of iodine toxicity and thyroid damage. (6)

That means you can overdo it by eating some types of seaweed. For example, kombo (or kelp) can contain 8000 μg iodine per gram. (7)

It doesn’t mean that you having a sushi role will overdose you on Iodine. You may indeed need a larger dose depending on your constitution. It’s simply best to refer to your health practitioner rather than than relying totally on “Dr Search engine” or “Dr AI”

4. Vitamin D

Vitamin D is fat-soluble and so can accumulate in your body if you take too much. High vitamin D can cause high calcium levels and mineral deposits in soft tissues. (8)

The safe upper limit is set at 4000 IU/day. However some people with serious conditions such as COVID patients have taken much higher doses (under supervision) without adverse effects. (9)

At home, stick to 800-1000 IU/day. Otherwise it’s best to only take higher doses under guidance by a health professional.

5. Selenium

Thinning hair, diarrhoea, and nausea are amongst other things signs of selenium toxicity, i.e. selenosis. (10) This can happen when you eat too much selenium - usually via supplements or Brazil nuts.

The amount of selenium in Brazil nuts varies a lot depending where they’re grown. (11) One Brazil nut could contain up to 3x more selenium than you need - or hardly any.

Best advice is to eat around than 1-2 Brazil nuts/day. As a starting point as the founding father of medicine, Hippocrates stated, “Let food be thy medicine and medicine be thy food”

6. Calcium

70% of us are believed to be lactose intolerant, so most people need to get calcium somewhere other than dairy. (12)

Yet, taking high-dose calcium is a problem because it stops you absorbing iron and zinc. (13)

Eating a variety of foods in moderation is the key, as Calcium-rich leafy greens contain oxalic acid, an anti-nutrient which blocks calcium absorption. Again, this makes calcium oxalate crystals and increases your risk of kidney stones. (14) This is not to say to avoid leafy greens, that just wouldn’t be right. It’s just that people seem to more and more go overboard even on good things.

7. Vitamin A

Have you heard the stories of kids ending up in a hospitals emergency department after eating too many multivitamin gummies?

Vitamin A is a fat-soluble nutrient which causes toxicity if you have too much. (15) It’s found in animal foods such as meat, fish and dairy.

But there is a safer way to get vitamin A. You can convert beta-carotene. (16)

By eating foods such as carrots and sweet potatoes, your body only converts what it needs to vitamin A. Now that’s nature for you!

Best to avoid supplements with high doses of beta-carotene. High doses are linked to reduced conversion to vitamin A and increased cancer risk. (17,18)

The bottom line

Taking to many vitamins can cause your body more harm than good.

Even high doses of water-soluble vitamins can trigger odd symptoms.

The simple fact is that it’s best to get your Vitamins from whole-foods. Always eat things in moderation and be aware of your body, as it will be communicating with you every time you eat something even if subtly.

References

1. Dietary Reference Intakes for Vitamin C, Vitamin E, Selenium, and Carotenoids. 2000 [cited 2022 Jul 14]; Available from: https://pubmed.ncbi.nlm.nih.gov/25077263/

2. Knight J, Madduma-Liyanage K, Mobley JA, Assimos DG, Holmes RP. Ascorbic acid intake and oxalate synthesis. Urolithiasis [Internet]. 2016 Aug [cited 2022 Jul 14];44(4). Available from: https://pubmed.ncbi.nlm.nih.gov/27002809/

3. Ferraro PM, Curhan GC, Gambaro G, Taylor EN. Total, Dietary, and Supplemental Vitamin C Intake and Risk of Incident Kidney Stones. Am J Kidney Dis [Internet]. 2016 Mar [cited 2022 Jul 14];67(3). Available from: https://pubmed.ncbi.nlm.nih.gov/26463139/

4. Kernan KF, Carcillo JA. Hyperferritinemia and inflammation. Int Immunol [Internet]. 2017 Nov 1 [cited 2022 Jul 14];29(9). Available from: https://pubmed.ncbi.nlm.nih.gov/28541437/

5. Brissot P, Pietrangelo A, Adams PC, de Graaff B, McLaren CE, Loréal O. Haemochromatosis. Nature reviews Disease primers [Internet]. 2018 Apr 5 [cited 2022 Jul 14];4. Available from: https://pubmed.ncbi.nlm.nih.gov/29620054/

6. Koukkou EG, Roupas ND, Markou KB. Effect of excess iodine intake on thyroid on human health. Minerva Med [Internet]. 2017 Apr [cited 2022 Jul 14];108(2). Available from: https://pubmed.ncbi.nlm.nih.gov/28079354/

7. Southern AP, Jwayyed S. Iodine Toxicity. 2022 Jan [cited 2022 Jul 14]; Available from: https://pubmed.ncbi.nlm.nih.gov/32809605/

8. Rizzoli R. Vitamin D supplementation: upper limit for safety revisited? Aging Clin Exp Res [Internet]. 2021 Jan [cited 2022 Jul 14];33(1). Available from: https://pubmed.ncbi.nlm.nih.gov/32857334/

9. Annweiler C, Beaudenon M, Gautier J, Gonsard J, Boucher S, Chapelet G, et al. High-dose versus standard-dose vitamin D supplementation in older adults with COVID-19 (COVIT-TRIAL): A multicenter, open-label, randomized controlled superiority trial. PLoS Med [Internet]. 2022 May 31 [cited 2022 Jul 14];19(5). Available from: https://pubmed.ncbi.nlm.nih.gov/35639792/

10. Alexander J. Selenium. Novartis Found Symp [Internet]. 2007 [cited 2022 Jul 14];282. Available from: https://pubmed.ncbi.nlm.nih.gov/17913229/

11. Ec SJ, Wadt LHO, Silva KE, Lima RMB, Batista KD, Guedes MC, et al. Natural variation of selenium in Brazil nuts and soils from the Amazon region. Chemosphere [Internet]. 2017 Dec [cited 2022 Jul 14];188. Available from: https://pubmed.ncbi.nlm.nih.gov/28923728/

12. Szilagyi A, Ishayek N. Lactose Intolerance, Dairy Avoidance, and Treatment Options. Nutrients [Internet]. 2018 Dec 15 [cited 2022 Jul 14];10(12). Available from: https://pubmed.ncbi.nlm.nih.gov/30558337/

13. Jayalakshmi S, Platel K. Supplemental levels of iron and calcium interfere with repletion of zinc status in zinc-deficient animals. Food Funct [Internet]. 2016 May 18 [cited 2022 Jul 14];7(5). Available from: https://pubmed.ncbi.nlm.nih.gov/27101872/

14. Mitchell T, Kumar P, Reddy T, Wood KD, Knight J, Assimos DG, et al. Dietary oxalate and kidney stone formation. Am J Physiol Renal Physiol [Internet]. 2019 Mar 1 [cited 2022 Jul 14];316(3). Available from: https://pubmed.ncbi.nlm.nih.gov/30566003/

15. Bendich A, Langseth L. Safety of vitamin A. Am J Clin Nutr [Internet]. 1989 Feb [cited 2022 Jul 14];49(2). Available from: https://pubmed.ncbi.nlm.nih.gov/2492745/

16. Bohn T, Desmarchelier C, El SN, Keijer J, van Schothorst E, Rühl R, et al. β-Carotene in the human body: metabolic bioactivation pathways - from digestion to tissue distribution and excretion. Proc Nutr Soc [Internet]. 2019 Feb [cited 2022 Jul 14];78(1). Available from: https://pubmed.ncbi.nlm.nih.gov/30747092/

17. Novotny JA, Harrison DJ, Pawlosky R, Flanagan VP, Harrison EH, Kurilich AC. Beta-carotene conversion to vitamin A decreases as the dietary dose increases in humans. J Nutr [Internet]. 2010 May [cited 2022 Jul 14];140(5). Available from: https://pubmed.ncbi.nlm.nih.gov/20237064/

18. Druesne-Pecollo N, Latino-Martel P, Norat T, Barrandon E, Bertrais S, Galan P, et al. Beta-carotene supplementation and cancer risk: a systematic review and metaanalysis of randomized controlled trials. International journal of cancer [Internet]. 2010 Jul 1 [cited 2022 Jul 14];127(1). Available from: https://pubmed.ncbi.nlm.nih.gov/19876916/