Supplement–Drug Interactions
Most supplement–drug combinations are harmless, but a few genuinely matter — and people often don’t mention supplements to their doctor. This guide covers the interactions worth knowing: bleeding risk, absorption/spacing, and the handful of supplements (like St John’s Wort) that interfere with many medications. It’s general information, not a substitute for your pharmacist.
Last reviewed Jun 24, 2026 · Evidence-based — every ingredient links to its underlying studies.
This is general guidance — check with your pharmacist
Interactions depend on your exact medications, doses, and health. Always tell your doctor and pharmacist about every supplement you take (most people don’t), and check before starting anything new if you’re on prescription medication. When in doubt, your pharmacist is the fastest, free expert.
- Tell your pharmacist about all supplements
- Space minerals away from thyroid meds & some antibiotics
- Pause bleeding-risk supplements before surgery (ask your surgeon)
- Combine St John’s Wort with prescription meds
- Add fish oil/ginkgo/garlic on top of blood thinners without advice
- Assume "natural" means "no interactions"
Key point: A few combinations are genuinely risky (bleeding, St John’s Wort); many others are just about spacing doses apart.
The interactions that matter most
Grouped by what to do. "Separate" = a real clash or spacing issue; "Caution" = monitor / get advice.
Don’t combine without medical advice.
It induces drug-metabolising enzymes and can weaken birth control (documented unplanned pregnancies), blood thinners, immunosuppressants (transplant rejection), and some HIV/cancer drugs — and it ALSO adds to serotonergic drugs (SSRIs, SNRIs, triptans, tramadol), risking serotonin syndrome. One of the highest-impact supplement interactions there is.
Get advice; watch for bleeding.
High-dose fish oil can add to the anticoagulant effect of warfarin/DOACs and antiplatelets. Usually manageable, but tell your clinician.
Avoid combining without advice.
Ginkgo has antiplatelet activity and can increase bleeding risk alongside anticoagulants/antiplatelets.
Use culinary amounts; flag supplements.
Concentrated garlic supplements can add to bleeding risk with anticoagulants.
Keep intake steady, don’t spike it.
Vitamin K directly opposes warfarin. Sudden changes (big doses or stopping) destabilise your INR — consistency matters more than avoidance.
Space at least 4 hours apart.
Calcium, iron, magnesium, and antacids all bind levothyroxine in the gut and cut its absorption, under-treating your thyroid. Take thyroid meds on an empty stomach, minerals/antacids hours later.
Space 2–6 hours apart.
Minerals chelate quinolone and tetracycline antibiotics, reducing their absorption and effectiveness. Separate the doses.
Don’t add potassium without advice.
These blood-pressure drugs raise potassium; extra potassium (incl. "salt substitutes") can push it dangerously high. The risk is greatest if you also take a potassium-sparing diuretic (spironolactone, amiloride) or have reduced kidney function.
Get advice; watch blood sugar.
Berberine inhibits CYP3A4 and P-glycoprotein, so it tends to RAISE levels of drugs they clear (some statins, cyclosporine), and it adds to glucose-lowering drugs — it can stack with metformin/insulin.
Before surgery
Many surgeons ask you to stop bleeding-risk supplements — fish oil, ginkgo, garlic, vitamin E, high-dose ginseng — roughly 1–2 weeks beforehand, and St John’s Wort because it affects anaesthetic drug metabolism. Always follow your own surgical team’s instructions.
Sources & further reading
Per-supplement interaction detail lives on each page; our interaction checker covers supplement-vs-supplement pairs.
Common questions
Which supplements interact with blood thinners?
The bleeding-risk group: fish oil (high dose), ginkgo, concentrated garlic, vitamin E, and high-dose ginseng can add to anticoagulants/antiplatelets. Vitamin K is the opposite problem — it opposes warfarin, so keep intake steady. Always tell your clinician.
Can I take supplements with antibiotics?
Most are fine, but space calcium, iron, magnesium, and zinc several hours away from quinolone and tetracycline antibiotics — taken together they blunt the antibiotic. A probiotic a few hours apart is commonly used during a course.
Why is St John’s Wort such a problem?
It speeds up the enzymes that clear many drugs, lowering their levels — there are documented cases of unplanned pregnancy (failed birth control) and transplant rejection, and it weakens blood thinners and some HIV/cancer drugs. It can also add to serotonergic drugs (SSRIs, SNRIs, triptans) and trigger serotonin syndrome. Avoid it entirely on prescription medication unless your doctor approves.
Do I really need to tell my doctor about supplements?
Yes — most people don’t, and that’s exactly how avoidable interactions happen. Bring the actual bottles or a list to appointments; your pharmacist can screen them in minutes.
Educational guidance, not medical advice. Evidence and safety details for each option live on its individual page; see a clinician for prescription treatments or persistent problems.
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