Drug interactions
Fish oil, vitamin K, St. John's Wort, magnesium, and calcium all interact with common medications. Learn the most clinically significant interactions and how to check your own stack.
High-impact supplement–drug interactions include St. John’s wort with contraceptives and transplant drugs, vitamin K with warfarin, high-dose fish oil and vitamin E with bleeding-risk medications, magnesium with some antibiotics and muscle relaxants, and calcium with levothyroxine absorption. This is not an exhaustive list—always provide a complete supplement list to prescribers and pharmacists. NutriAudit is a planning tool; it does not replace pharmaceutical interaction databases or professional review.
| Supplement | Drug class | Mechanism | Action |
|---|---|---|---|
| St. John’s wort | Many substrates | Enzyme induction | Avoid unless cleared |
| Vitamin K | Warfarin | Antagonism balance | Consistency + monitoring |
| Fish oil (high dose) | Anticoagulants | Bleeding | INR/bleeding vigilance |
| Calcium | Levothyroxine | Binding/absorption | Separate dosing hours |
Source: NIH ODS interaction summaries; verify with prescribing clinicians.
Bring bottles to appointments. Labels beat memory—especially proprietary blends.
Update after any hospitalization. New drugs change interaction risk overnight.
Pharmacist medication review. Use yearly comprehensive reviews if you take 5+ drugs.
Never trust “natural = safe.” Herb–drug interactions are common and serious.
Sleep stacks combine melatonin, valerian, magnesium, and sometimes CBD—each interacts with sedatives differently.
NutriAudit helps enumerate products so clinicians can run formal interaction checks.
Some interactions change drug levels (induction, inhibition, binding). Others stack side effects (bleeding, sedation, QT risk) even when levels look unchanged. Both belong on your review list.
Grapefruit is famous, but goldenseal, St. John’s wort, and high-dose CBD also perturb metabolism in select pathways.
Bring bottles or PDFs of supplement facts, not brand names from memory. Ask explicitly, “Does anything here affect my warfarin/metformin/antidepressant?”
Community pharmacies often offer medication reviews—use them when your stack grows beyond three daily products.
Good ones do—bring complete lists.
Grapefruit affects drugs via CYP3A4; some supplements also induce or inhibit enzymes.
Some doxycycline-like drugs bind calcium—follow pharmacy instructions.
Levothyroxine absorption is sensitive—follow prescriber timing rules.
Use NutriAudit to audit your full stack for hidden overlaps.
Audit your supplement stackDisclaimer: NutriAudit is a decision-support tool designed to help you review your supplement stack for potential duplicate, conflicting, or excessive ingredients. It is not a substitute for professional medical advice, diagnosis, or treatment. Always consult a qualified healthcare provider before making changes to your supplement routine, especially if you are pregnant, nursing, taking medications, or have a medical condition.
Based on reference standards from FDA, EFSA, TGA, and MHLW.
Last updated: 2026-04-07 · Data sourced from FDA Dietary Reference Intakes, EFSA Scientific Opinions, and NIH Office of Dietary Supplements where applicable.