CitedDose

Supplement interaction checker

Most supplement problems are not dangerous reactions but absorption conflicts and timing: calcium blocks iron and some antibiotics, zinc competes with copper, and vitamin K counteracts warfarin. A few are serious: St John's wort reduces the effect of many prescription drugs, and high-dose vitamin E, fish oil, ginkgo and garlic add to bleeding risk with blood thinners. Tick what you take to see the specific conflicts and how to space them.

Last reviewed: 2026-10-06

How this is calculated

Three kinds of interaction matter in practice.

1. Absorption conflicts (timing fixes them). Divalent minerals (calcium, iron, magnesium, zinc) compete with each other and bind certain drugs in the gut: levothyroxine, tetracycline and fluoroquinolone antibiotics, bisphosphonates. Spacing by 2 to 4 hours resolves most of these.

2. Pharmacodynamic overlap (dose matters). Vitamin E, fish oil, ginkgo and garlic each have mild antiplatelet effects. Alone and at normal doses they are safe; stacked with warfarin, clopidogrel or aspirin, bleeding risk rises. Vitamin K directly opposes warfarin, so the goal is a constant intake, not zero.

3. Enzyme induction (the dangerous one). St John's wort induces CYP3A4 and P-glycoprotein, lowering blood levels of roughly half of all prescription drugs: contraceptives, immunosuppressants, HIV antivirals, anticoagulants, statins, some antidepressants. It also adds serotonergic activity to SSRIs. It is the single supplement most pharmacists would ask about first.

Sources for each pairing are the NIH Office of Dietary Supplements fact sheets, the Natural Medicines database interaction monographs, and the primary case reports and trials listed below. Severity ratings follow the convention: serious (avoid or requires prescriber input), moderate (manage with timing or monitoring), minor (awareness only).

Frequently asked questions

Which supplements should not be taken together?

The common absorption conflicts are calcium with iron, zinc with iron, and zinc with copper (long-term high zinc depletes copper). None are dangerous; separate them by about 2 hours or take at different meals.

What supplements interact with blood thinners?

Vitamin K (counteracts warfarin), and high-dose vitamin E, fish oil, ginkgo and garlic extract (add to bleeding risk). St John's wort lowers the levels of warfarin and the newer anticoagulants. Tell your anticoagulation clinic about every supplement.

Can I take vitamins with levothyroxine?

Not at the same time. Calcium, iron and magnesium bind levothyroxine and reduce absorption. Take levothyroxine on an empty stomach first thing, and minerals at least 4 hours later.

Is St John's wort safe with other medication?

Usually not. It reduces the effect of oral contraceptives, immunosuppressants, HIV drugs, anticoagulants, digoxin, some statins and more, and it can cause serotonin syndrome with antidepressants. Check with a pharmacist before taking it with any prescription.

Do I need to separate magnesium and calcium?

Only if taking large doses of both at once (over about 250 mg each); the competition is modest. Many combined products exist. Spacing them is a refinement, not a requirement.

Does this checker cover my prescription?

It covers common drug classes, not every medicine. Treat a clear result as a starting point and confirm with a pharmacist, who can run your full list through a clinical database.

Sources

  1. NIH Office of Dietary Supplements. Dietary supplement fact sheets (calcium, iron, zinc, magnesium, vitamin D, vitamin E, vitamin K, omega-3) (interaction sections of each sheet)
  2. Henderson L et al. St John's wort (Hypericum perforatum): drug interactions and clinical outcomes. Br J Clin Pharmacol. 2002;54(4):349-56
  3. Skelin M et al. Factors affecting gastrointestinal absorption of levothyroxine: a review. Clin Ther. 2017;39(2):378-403
  4. Lönnerdal B. Calcium and iron absorption: mechanisms and public health relevance. Int J Vitam Nutr Res. 2010;80(4-5):293-9
  5. Bent S et al. Spontaneous bleeding associated with ginkgo biloba: a case report and systematic review. J Gen Intern Med. 2005;20(7):657-61
  6. Neuvonen PJ. Interactions with the absorption of tetracyclines. Drugs. 1976;11(1):45-54

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Educational information only, not medical advice. Results are population estimates from published studies and may not apply to you.