Interactions Between Supplements and Medications: What You Need to Know
A persistent misconception is that what is natural is automatically risk-free. This is a dangerous oversimplification. Medicinal plants and natural supplements are composed of biologically active molecules, and it is precisely because they act on the body that they can, in certain contexts, interact with medications. These interactions are not frequent, but some are clinically significant. Knowing the main ones, and knowing when to consult, is an essential part of a responsible natural health approach. Our natural health advisors systematically address this topic with their clients.
Why Interactions Exist
Interactions between supplements and medications occur according to three main mechanisms. The first is pharmacokinetic: the supplement modifies the absorption, distribution, metabolism or elimination of the medication, which changes the active amount of it in the blood. The second is pharmacodynamic: the supplement and the medication have similar effects that add up (potentiation) or opposite effects that cancel each other out (antagonism). The third is physicochemical: the two substances form a complex in the digestive tract that reduces the absorption of one or the other.
Several studies have estimated that fewer than half of supplement users spontaneously disclose their use to their doctor. This phenomenon illustrates why interactions go unnoticed in many cases. This is not a problem of bad faith: it is often because people do not realize that their vitamin E or ginkgo extract can have an impact on their treatment.
The Most Important Interactions to Know
St. John's Wort (Hypericum perforatum) and Numerous Medications
This is the most documented and most serious interaction in the field of medicinal plants. St. John's wort is used to support mood and reduce mild to moderate depressive symptoms. It is also a powerful inducer of cytochrome P450 (CYP3A4), a hepatic enzyme system that metabolizes a very large proportion of common medications.
In practice, St. John's wort accelerates the breakdown of many medications, reducing their concentration in the blood and their effectiveness: oral contraceptives (risk of unwanted pregnancy), antivirals (HIV treatment), immunosuppressants (after organ transplant), anticoagulants, certain SSRI-type antidepressants (risk of serotonin syndrome). This interaction is officially documented by Health Canada and European regulatory agencies. If you take a chronic medication, St. John's wort should always be the subject of a discussion with your doctor or pharmacist.
Omega-3s and Anticoagulants or Antiplatelets
Omega-3s (EPA and DHA) have natural blood-thinning properties. Taken alone at reasonable doses (1 to 2 g of EPA+DHA per day), they are safe for the vast majority of people. On the other hand, combined with anticoagulants like warfarin (Coumadin) or rivaroxaban (Xarelto), or with antiplatelets like aspirin or clopidogrel (Plavix), they can increase the risk of bleeding.
According to EFSA, the safety limit for omega-3s is 5 g of EPA+DHA per day, but interactions with anticoagulants can occur at much lower doses in people already on treatment. It is also recommended to stop omega-3s at least two weeks before a surgical procedure, as well as ginkgo biloba, turmeric and garlic which have similar properties.
Magnesium and Antibiotics
Magnesium forms insoluble complexes with certain classes of antibiotics, notably quinolones (ciprofloxacin, levofloxacin) and tetracyclines (doxycycline). These complexes are not absorbed, which reduces the effectiveness of the antibiotic and increases the risk of treatment failure and bacterial resistance. The solution is simple: space out magnesium intake by at least two hours before or after your antibiotic. This precaution also applies to calcium, zinc and iron.
Vitamin K and Warfarin-Type Anticoagulants
Warfarin works by blocking the action of vitamin K in blood coagulation. Any variable intake of vitamin K (foods or supplements) can therefore modify the effectiveness of this medication. Vitamin K2 (MK-7), increasingly recommended for bone health with vitamin D, is affected by this interaction. If you take warfarin, maintain vitamin K intake as constant as possible and report any supplementation changes to your doctor or anticoagulation therapist.
Turmeric and Anti-Inflammatory Medications or Anticoagulants
Turmeric, and more specifically curcumin, has well-documented anti-inflammatory properties. It can potentiate the effect of NSAIDs (ibuprofen, naproxen) and increase the risk of digestive bleeding. At high doses, it can also modify the hepatic metabolism of certain medications via CYP450 enzymes. At normal dietary doses, these interactions are negligible. It is in concentrated supplementation (extracts standardized to 95% curcumin, liposomal formulas) that vigilance is required.
Ginkgo Biloba and Antiplatelets or Anticoagulants
Ginkgo biloba is used for memory, circulation and cognitive support. Antiplatelet properties have been observed in vitro and in certain case reports, though controlled clinical studies have not demonstrated a significant interaction with aspirin or warfarin. As a precaution, it is generally recommended to stop ginkgo biloba two weeks before any surgery and to inform your doctor if you are taking anticoagulant or antiplatelet medications.
Quick Reference Table
| Natural Supplement | Potential Interaction to Report to Your Doctor |
|---|---|
| St. John's wort | Oral contraceptives, antivirals, immunosuppressants, SSRIs, anticoagulants |
| High-dose omega-3s | Warfarin, Xarelto, aspirin, Plavix, NSAIDs |
| Magnesium (high dose) | Quinolones and tetracyclines (space out by at least 2 hours) |
| Vitamin K2 (MK-7) | Warfarin and other coumarin anticoagulants |
| Turmeric (concentrated extract) | NSAIDs, anticoagulants, medications metabolized by CYP450 |
| Ginkgo biloba | Anticoagulants, antiplatelets, aspirin |
| Iron (supplement) | Quinolones, tetracyclines, thyroid medications (space out by 2 hours) |
| High-dose zinc | Antibiotics (quinolones), medications for rheumatoid arthritis |
Frequently Asked Questions
Can my pharmacist check for interactions with my supplements?
Absolutely, and it is one of the best habits to develop. Quebec pharmacists have access to drug interaction databases that now include many natural health products. Bring a complete list of your supplements (including doses) to each prescription renewal.
Can a natural supplement sold over the counter really not interact with my medications?
The accessibility of a product without a prescription does not mean the absence of interactions. Prescription medications themselves interact with each other. Natural supplements contain biologically active molecules, which is why they have effects. It simply needs to be taken into account with the same seriousness as for medications.
Should I stop all my supplements before surgery?
Systematically inform your surgeon and the anesthesia team of all your supplements and medications. Generally, supplements with anticoagulant or antiplatelet properties (omega-3s, ginkgo, concentrated turmeric, garlic, high-dose vitamin E) should be stopped two weeks before an elective procedure. Other supplements can be maintained according to clinical assessment.
At La Boite à Grains, our graduate naturopaths are trained to take into account your complete health profile, including your medical treatments, before recommending a supplement. Make an appointment with our advisors in our Gatineau store or explore our selection of natural supplements online. To learn more about responsible supplementation, consult our natural health articles.
References
- St. John's Wort (Hypericum perforatum), Drug Interactions. Health Canada, Health Product InfoWatch. Government of Canada. 2020. https://www.canada.ca/en/health-canada/services/drugs-health-products/medeffect-canada/health-product-infowatch/october-2020.html
- Natural Product–Drug Interactions. Prevention Watch, Montreal Heart Institute. Université de Montréal. Updated 2024. https://observatoireprevention.org/en/2019/04/01/interactions-entre-les-produits-naturels-et-les-medicaments/
- Borrelli F. and Izzo A.A. Herb-drug interactions with St John's wort (Hypericum perforatum): an update on clinical observations. The AAPS Journal, vol. 11, no. 4, p. 710–727. 2009.
- EFSA Panel on Dietetic Products, Nutrition and Allergies. Scientific Opinion on the Tolerable Upper Intake Level of eicosapentaenoic acid (EPA), docosahexaenoic acid (DHA) and docosapentaenoic acid (DPA). EFSA Journal. 2012, reaffirmed 2024–2026. https://www.efsa.europa.eu/en/press/news/120727
- Bone K.M. Potential interaction of Ginkgo biloba leaf with antiplatelet or anticoagulant drugs: what is the evidence? Molecular Nutrition & Food Research, vol. 52, no. 7, p. 764–771. 2008.
- Izzo A.A. and Ernst E. Interactions between herbal medicines and prescribed drugs: an updated systematic review. Drugs, vol. 69, no. 13, p. 1777–1798. 2009.
- Schwalfenberg G.K. Vitamins K1 and K2: the emerging group of vitamins required for human health. Journal of Nutrition and Metabolism, vol. 2017, article 6254836. 2017.
- Hu Z. et al. Pharmacokinetic interactions of curcuminoids with conventional drugs: a review. Journal of Ethnopharmacology, vol. 209, p. 193–213. 2017.
- Stargrove M.B., Treasure J. and McKee D.L. Herb, Nutrient, and Drug Interactions: Clinical Implications and Therapeutic Strategies. Mosby Elsevier. 2008.