Multivitamin Side Effects: Nausea, Constipation and Getting Too Much of a Nutrient
Documented side effects of nutrients found in multivitamins, including iron-related nausea and constipation, upper-level cautions, and when to ask a clinician or pharmacist.
Content last updated . This page has not been medically reviewed.
Urgent first: overdose and children
If a child, or anyone, may have swallowed a large amount of a multivitamin that contains iron, call Poison Help at 1-800-222-1222 or 911 (US) right away. MedlinePlus lists vomiting blood, black or bloody stools, drowsiness, convulsions and shock among iron overdose symptoms, and says symptoms may ease and then return a day or later. [1] Severe overdose is most dangerous for children who eat adult multivitamins. [1] Steps and other countries: first steps and Poison Help, poison help by country.The short answer
NCCIH says taking a multivitamin is unlikely to pose any health risks. [2] The documented problems come mostly from specific nutrients, especially iron, and from taking more than you need once several products are added together. Proper evidence not found for how often nausea or constipation occurs with multivitamins as a product type: the trial abstracts, NHS pages, NCCIH and MedlinePlus pages we read give no rates. Overview: multivitamins.Nutrient by nutrient: what official sources describe
Evidence label for the rows below: established nutritional role for the nutrient, with the effect shown being excess.| Nutrient | Effect described by the NHS (UK, 2020 unless noted) |
|---|---|
| Iron | Doses over 20 mg can cause constipation, feeling sick, being sick and stomach pain; very high doses can be fatal, especially in children [3] |
| Iron tablets for anemia | Possible side effects include constipation or diarrhea, tummy pain, heartburn, feeling sick and darker stools; the NHS (2024) says taking tablets with or soon after food can reduce the chance of side effects [4] |
| Vitamin C | More than 1,000 mg a day can cause stomach pain, diarrhea and flatulence, which should go away after stopping [5] |
| Calcium | More than 1,500 mg a day could lead to stomach pain and diarrhea [6] |
| Magnesium | More than 400 mg for a short time can cause diarrhea [7] |
| Zinc | High doses reduce copper absorption, which can lead to anemia and weakened bones [7] |
| Copper | High doses could cause stomach pain, sickness and diarrhea, and liver and kidney damage if taken for a long time [7] |
| Vitamin B6 | 200 mg or more a day can cause loss of feeling in the arms and legs, which in a few cases has been permanent [8] |
| Niacin (nicotinic acid) | High doses can cause skin flushes; high doses for a long time could lead to liver damage [8] |
| Folic acid | More than 1 mg can mask the symptoms of B12 deficiency [8] |
| Vitamin A | An average above 1.5 mg (1,500 µg) a day over many years may affect bones; the NHS notes many multivitamins contain it [9] |
| Vitamin D | Too many supplements over a long period can cause too much calcium in the body, weakening bones and harming kidneys and heart [10] |
| Selenium | Too much causes selenosis, which in its mildest form can lead to loss of hair and nails [7] |
| Beta-carotene | Supplements increased lung cancer risk in people who smoke or were heavily exposed to asbestos [7] |
These are levels for adults in the UK. They are not targets and do not apply to every country, age or product; EU upper levels are on multivitamins. US figures from NIH Office of Dietary Supplements could not be verified, so none are quoted.
Getting too much of a nutrient
MedlinePlus says many people think that if some is good a lot is better, but high doses of certain vitamins can be toxic, and that vitamins A, D, E and K can build up in the body. [11] Multivitamins, other supplements, fortified foods and drinks all add together. See doubling up on nutrients and understanding vitamins.Large pills, pregnancy and other groups
- Swallowing: NCCIH cites a 2015 study estimating about 23,000 US emergency room visits a year from dietary supplements, many by older adults with swallowing problems from large vitamin pills. [2] See supplements for people who have trouble swallowing pills.
- Pregnancy: the NHS (2015) says high vitamin A can harm the baby and advises a multivitamin made for pregnancy or without vitamin A. [12]
- Children: the NHS warns that too much of some vitamins can be harmful and advises not giving two supplements at once. [13]
- Kidney or liver conditions: the NHS notes long-term high copper can damage the liver and kidneys and that older people are advised not to take potassium supplements unless a doctor says so, because their kidneys may remove potassium less well. [7] Ask a clinician first; see minerals and kidney disease and liver safety.
- Medicines and surgery: supplements may interact with medicines or matter before surgery. [2] Vitamin K can reduce the effect of warfarin. [14] See medication interactions and supplements before surgery.
If you have side effects
NCCIH advises that if you have side effects, you stop the supplement and contact your health care provider. [2] Do not stop or change a prescribed medicine without asking your prescriber or pharmacist. Report problems via side effects, allergic reactions and reporting. Check labels against quality testing and certifications: the FDA is not authorized to approve supplements for safety before they are marketed. [15]Frequently asked questions
Why does my multivitamin make me feel sick?
The sources we read name iron as a documented cause of feeling sick and stomach pain at higher amounts. [3] They do not explain individual cases; ask a pharmacist or clinician.
Can a multivitamin cause constipation?
Iron can. The NHS lists constipation among side effects of doses over 20 mg. [3]
Can I take too much from a multivitamin?
Yes, especially when you add other products. The NHS gives upper guidance for several vitamins and minerals, such as 10 mg of B6 a day in supplements unless a doctor advises. [8]
Does taking it with food help?
For iron tablets given for anemia, the NHS says taking them with or soon after food may reduce side effects. [4] Proper evidence not found in the sources we read for multivitamins generally; ask a pharmacist.
Are multivitamins safe in pregnancy?
Products made for pregnancy exist; the NHS advises picking one made for pregnancy or without vitamin A. [12] Ask your midwife, clinician or pharmacist.
References
- MedlinePlus: Iron overdose (reviewed 14 October 2025). Read 2026-10-23.
- NCCIH: Using Dietary Supplements Wisely. Read 2026-10-23.
- NHS: Iron. Read 2026-10-23.
- NHS: Iron deficiency anaemia. Read 2026-10-23.
- NHS: Vitamin C. Read 2026-10-23.
- NHS: Calcium. Read 2026-10-23.
- NHS: Other vitamins and minerals (beta-carotene, copper, magnesium, potassium, selenium, zinc and others). Read 2026-10-23.
- NHS: B vitamins and folic acid. Read 2026-10-23.
- NHS: Vitamin A. Read 2026-10-23.
- NHS: Vitamin D. Read 2026-10-23.
- MedlinePlus: Vitamins (encyclopedia article, reviewed 21 January 2025). Read 2026-10-23.
- NHS: Vegetarian and vegan diets during pregnancy (information reviewed April 2015). Read 2026-10-23.
- NHS: Vitamins for children. Read 2026-10-23.
- NIA: Vitamins and Minerals for Older Adults (content reviewed 2 January 2021). Read 2026-10-23.
- FDA: Information for Consumers on Using Dietary Supplements. Read 2026-10-23.
Education, not medical advice. This page is for general information. It does not diagnose or treat any condition and is not a substitute for care from a qualified health professional. Natural does not automatically mean safe. Ask a clinician or pharmacist before changing your care, especially if you take medication, are pregnant or breastfeeding, or have a health condition.