Which Vitamins and Supplements Should Not Be Taken Together?
Last updated: August 2026 · Reviewed by the FactoWiki Editorial Team for clarity and source accuracy
The main conflicts are calcium with iron and zinc, zinc with copper at high doses, and fibre with anything prescribed. Separate those by two hours. The interactions that matter more are supplement-to-medication, and vitamin K with warfarin is the one that can genuinely land someone in hospital.
Key takeaways
- Calcium blocks iron and zinc — separate by two hours.
- Vitamin K and warfarin is the interaction that most often causes real harm.
- Fibre supplements bind medications; separate by two hours minimum.
The mineral competitions
Calcium competes with iron, zinc and magnesium for absorption — separate by two hours, and four hours from levothyroxine. High-dose zinc taken long term depletes copper, causing anaemia and neurological problems, which is why good zinc products include a small amount of copper. Iron and zinc compete when taken together at high doses.
The supplement-drug interactions that matter most
Vitamin K, including K2 in D3+K2 products, directly antagonises warfarin and can push an INR out of range. St John's wort induces CYP3A4 and reduces the effectiveness of oral contraceptives, some antidepressants, anticoagulants and tamoxifen. Curcumin, ginger, omega-3, Boswellia and vitamin E all add to bleeding risk with anticoagulants. Potassium with ACE inhibitors or ARBs risks hyperkalaemia.
The ones that stack unhelpfully
Multiple products containing the same nutrient push totals past upper limits without any single label looking excessive — vitamin A, vitamin D and vitamin B6 are the usual offenders, and B6 above 100 mg long term causes peripheral neuropathy. Add up every source including fortified foods and drinks.
Key ingredients to understand
The ingredients most relevant to this question, with what the evidence actually supports:
- Calcium Citrate — Calcium citrate absorbs without stomach acid, which makes it the right choice for older adults, anyone on a proton pump inhibitor, and anyone…
- Iron Bisglycinate — Iron bisglycinate is chelated iron with markedly better tolerability than ferrous sulphate — far less constipation and nausea — at comparable…
- Zinc Glycinate — Zinc glycinate (bisglycinate) is zinc chelated to glycine, which improves absorption and markedly reduces the nausea that zinc commonly causes.…
- Vitamin K2 MK-7 — MK-7 is the long-chain form of vitamin K2 with a half-life measured in days rather than hours, making once-daily dosing practical. Its bone…
Frequently asked questions
Can I take a multivitamin and separate minerals?
You can, but total each nutrient across everything. Duplication is the commonest problem.
What about fat-soluble vitamins together?
A, D, E and K taken together with a fatty meal is fine, and they compete only marginally.
Should I tell my doctor what I take?
Yes, and bring the actual bottles. Supplement-drug interactions are common and under-reported.
Related on FactoWiki
- All health answers
- Ingredient evidence guides
- Calcium Citrate — ingredient guide
- Iron Bisglycinate — ingredient guide
- How we review
This article is general information, not medical advice. FactoWiki may earn a commission from links on product review pages (never on comparisons). Always check with a qualified healthcare professional about your own situation.