For most people, a standard multivitamin taken as directed causes no problems at all. When side effects do happen, they're usually mild and point to a specific ingredient. Here's how to tell the normal from the not-so-normal.
Written by Vita Globe Editorial Team | Reviewed against NIH Office of Dietary Supplements and FDA guidance
What are the side effects of multivitamins?
Most are mild and digestive: nausea, upset stomach or constipation, often linked to iron[1][2]. Problems are more likely when you stack supplements and exceed upper limits; 10% to 15% of multivitamin users had excessive intakes of vitamin A, iron or zinc[3]. One lesser-known issue: biotin can interfere with some lab tests[4].
Caveat: Severe symptoms, such as vomiting, dizziness, severe headache or blurred vision, aren't normal. Stop the supplement and get medical advice.
Sources: NIH ODS iron · Tolkien 2015 · NIH ODS multivitamins · FDA biotin · Verified 2026-09-27
In this article
What side effects are common and usually harmless?
- Nausea or upset stomach, especially on an empty stomach. Taking supplements with food helps[1]. We cover this in why multivitamins can make you nauseous.
- Constipation, most often from iron. In a meta-analysis of 43 trials, ferrous sulfate significantly increased digestive side effects compared with placebo[2].
These usually settle with food, a lower-iron formula, or time. If they don't, talk to your doctor.
Can a multivitamin affect blood test results?
It can, if it contains biotin. The FDA warns that biotin in dietary supplements can significantly interfere with certain lab tests and cause incorrect results, and it continues to receive reports of falsely low troponin results[4]. Troponin tests are used to help diagnose heart attacks.
According to the NIH, biotin interference has also caused falsely abnormal thyroid test results, in case reports involving people taking 10 to 300 mg of biotin a day[5]. That's far more than the 30 mcg adults need[5], but "hair, skin and nails" products can contain large amounts.
What to do: tell your doctor and the lab about every supplement you take, including multivitamins and beauty gummies, before any blood work.
What are the signs of taking too much?
Serious problems usually come from very high doses, often from stacking products, rather than from a standard multivitamin alone.
| Nutrient | Risk with excess |
|---|---|
| Vitamin A (preformed) | Very high doses can cause severe headache, blurred vision, nausea and dizziness; chronic excess can cause dry skin, joint pain and abnormal liver tests[6] |
| Vitamin B6 | Long-term very high doses can cause nerve damage (sensory neuropathy)[7] |
| Iron | High supplemental doses cause digestive side effects; accidental overdose is dangerous for children[1] |
To stay in safe territory, add up your totals across everything you take. Our guide to taking a multivitamin with other supplements walks through it.
When should you stop taking a multivitamin?
Stop and talk to a healthcare provider if you notice:
- Ongoing nausea, vomiting or stomach pain.
- Signs of an allergic reaction, such as rash, swelling or trouble breathing (seek urgent care).
- Numbness or tingling in your hands or feet, especially if you take high-dose B6.
- Any new symptom that started after you began a supplement.
Also keep all supplements, especially any containing iron, out of children's reach.
The honest summary
- Most multivitamin side effects are mild and digestive, often from iron.
- Taking it with food solves many problems.
- Biotin can skew some lab tests, so tell your doctor what you take.
- Serious toxicity usually comes from megadoses or stacking, not a standard multivitamin.
Our option
Vita Globe Adult Complete Multivitamin Gummies – Pouch
Each two-gummy serving provides vitamins A, C (60 mg), D (10 mcg), E, B6, folate, B12, biotin and pantothenic acid, plus zinc and iodine. The gummies are vegan, gluten-free and non-GMO, made without artificial colors or flavors, and come in resealable 14-count pouches, which is a week of servings at 2 gummies a day. It contains biotin, so mention it to your doctor before any blood work, as you would any supplement.
Frequently asked questions
Do multivitamins have side effects?
Most people have none. When they occur, they're usually mild digestive symptoms, often linked to iron[1].
Can multivitamins cause constipation?
Yes, particularly formulas with iron. Iron supplements are a well-documented cause of constipation and other digestive side effects[2].
Should I stop taking biotin before a blood test?
Ask your doctor. The FDA advises that biotin can interfere with some tests[4], and your clinician can tell you whether and when to pause it.
Can you take too many multivitamins?
Yes. Taking more than the labeled serving, or stacking several products, raises the risk of exceeding upper limits for nutrients like vitamin A, iron and zinc[3].
Sources
- Office of Dietary Supplements, National Institutes of Health. Iron: Fact Sheet for Health Professionals. Accessed September 27, 2026. ods.od.nih.gov
- Tolkien Z, Stecher L, Mander AP, et al. Ferrous sulfate supplementation causes significant gastrointestinal side-effects in adults: a systematic review and meta-analysis. PLoS One. 2015;10(2):e0117383. doi:10.1371/journal.pone.0117383
- Office of Dietary Supplements, National Institutes of Health. Multivitamin/mineral Supplements: Fact Sheet for Health Professionals. Updated July 17, 2024. ods.od.nih.gov
- U.S. Food and Drug Administration. Biotin Interference with Troponin Lab Tests: Assays Subject to Biotin Interference. Accessed September 27, 2026. fda.gov
- Office of Dietary Supplements, National Institutes of Health. Biotin: Fact Sheet for Health Professionals. Accessed September 27, 2026. ods.od.nih.gov
- Office of Dietary Supplements, National Institutes of Health. Vitamin A and Carotenoids: Fact Sheet for Health Professionals. Accessed September 27, 2026. ods.od.nih.gov
- Office of Dietary Supplements, National Institutes of Health. Vitamin B6: Fact Sheet for Health Professionals. Accessed September 27, 2026. ods.od.nih.gov
About the Vita Globe Editorial Team
The Vita Globe Editorial Team researches and writes our wellness content, working from peer-reviewed research, systematic reviews and guidance from bodies such as the NIH and the FDA. We cite our sources inline and update articles when the evidence changes.
These statements have not been evaluated by the Food and Drug Administration. This product is not intended to diagnose, treat, cure, or prevent any disease. This article is for informational purposes only and is not a substitute for medical advice. Talk to your healthcare provider before starting any new supplement, particularly if you are pregnant, nursing, taking medication, or caring for a child.
