Combination	Severity	What happens and what to do
Birth control pills + St. John's wort	Avoid combining	St. John's wort can make hormonal birth control less effective, leading to unplanned pregnancy.
Blood pressure medicines (ACE inhibitors, ARBs, spironolactone) + Potassium	Avoid combining	These medicines raise potassium; adding supplements can cause dangerously high potassium. Only use potassium if prescribed.
Digoxin + St. John's wort	Avoid combining	Can lower digoxin levels and reduce its effect.
SSRIs / antidepressants + St. John's wort	Avoid combining	Risk of serotonin syndrome, and it alters levels of many antidepressants.
SSRIs / antidepressants + 5-HTP	Avoid combining	Risk of serotonin syndrome.
St. John's wort + 5-HTP	Avoid combining	Both raise serotonin. Combined use may cause serotonin syndrome.
Statins + Red yeast rice	Avoid combining	Red yeast rice can contain monacolin K, the same compound as lovastatin. Combining doubles up on statin effects and side effects.
Warfarin + Vitamin K	Avoid combining	Vitamin K directly counteracts warfarin. Don't start or stop it without your prescriber; keep intake consistent.
Warfarin + St. John's wort	Avoid combining	Can reduce warfarin's effect and raise clot risk.
Ashwagandha + Melatonin	Use caution	Both can cause drowsiness. Start one at a time.
Ashwagandha + Valerian	Use caution	Additive sedative effects are possible.
Blood pressure medicines (ACE inhibitors, ARBs, spironolactone) + CoQ10	Use caution	CoQ10 may lower blood pressure slightly; monitor if combining.
Calcium + Iron	Use caution	Calcium reduces iron absorption. Take them at least 2 hours apart.
Calcium + Zinc	Use caution	High-dose calcium may lower zinc absorption. Take them at different times.
Calcium + Magnesium	Use caution	Very high doses compete for absorption. At normal doses, taking them together is fine.
Diabetes medicines (insulin, sulfonylureas) + Berberine	Use caution	Additive blood-sugar lowering; risk of hypoglycemia. Your doses may need adjusting.
Fish oil + Ginkgo	Use caution	Both may slightly affect blood clotting. Combined use may raise bleeding risk, especially before surgery.
Fish oil + Vitamin E	Use caution	High doses of both may add to bleeding risk. Normal doses are generally fine.
Immunosuppressants + Echinacea	Use caution	Echinacea may stimulate the immune system and counteract these medicines.
Immunosuppressants + Ashwagandha	Use caution	May stimulate immune activity; ask your transplant or rheumatology team.
Levothyroxine + Calcium	Use caution	Reduces levothyroxine absorption. Take at least 4 hours apart.
Levothyroxine + Iron	Use caution	Reduces levothyroxine absorption. Take at least 4 hours apart.
Levothyroxine + Magnesium	Use caution	May reduce absorption. Separate by at least 4 hours.
Levothyroxine + Biotin	Use caution	High-dose biotin can make thyroid blood tests look wrong. Stop it before blood work if your doctor advises.
Levothyroxine + Ashwagandha	Use caution	Ashwagandha may raise thyroid hormone levels.
Melatonin + Valerian	Use caution	Additive drowsiness. Don't drive or combine with alcohol.
Metformin + Berberine	Use caution	Both lower blood sugar; together they raise the risk of hypoglycemia. Only combine under medical supervision.
Psyllium fiber + Iron	Use caution	Fiber can reduce mineral absorption. Take minerals 2 hours apart from fiber supplements.
Psyllium fiber + Calcium	Use caution	Take calcium 2 hours apart from fiber supplements.
Sedatives or sleep medicines + Melatonin	Use caution	Additive drowsiness. Ask your doctor before combining.
Sedatives or sleep medicines + Valerian	Use caution	Additive sedation; avoid combining without medical advice.
Sedatives or sleep medicines + Ashwagandha	Use caution	May add to sedative effects.
Statins + Berberine	Use caution	Berberine affects liver enzymes that process some statins. Ask your pharmacist.
Tetracycline or quinolone antibiotics + Calcium	Use caution	Minerals bind these antibiotics. Take the antibiotic 2 hours before or 4–6 hours after.
Tetracycline or quinolone antibiotics + Magnesium	Use caution	Separate by at least 2 hours before or 4–6 hours after the antibiotic.
Tetracycline or quinolone antibiotics + Zinc	Use caution	Separate by at least 2 hours before or 4–6 hours after the antibiotic.
Tetracycline or quinolone antibiotics + Iron	Use caution	Separate by at least 2 hours before or 4–6 hours after the antibiotic.
Warfarin + Fish oil	Use caution	High doses may increase bleeding risk. Ask your doctor before taking more than 1–2 g/day.
Warfarin + Ginkgo	Use caution	May increase bleeding risk.
Warfarin + Vitamin E	Use caution	High doses may increase bleeding risk.
Warfarin + Turmeric	Use caution	High-dose turmeric or curcumin may increase bleeding risk.
Warfarin + Garlic	Use caution	Garlic supplements may increase bleeding risk.
Warfarin + CoQ10	Use caution	CoQ10 may reduce warfarin's effect. Ask your prescriber before starting.
Zinc + Iron	Use caution	Supplemental doses compete for absorption when taken together on an empty stomach. Separate them or take with food.
Zinc + Copper	Use caution	Long-term zinc above 40 mg/day can cause copper deficiency. Many zinc products add 1–2 mg copper for this reason.

Supplement	Warning
St. John's wort	Interacts with many medicines, including birth control, antidepressants and blood thinners.
Biotin	High doses can interfere with some lab tests, including thyroid and heart tests. Tell your doctor before blood work.
Green tea extract	Concentrated extracts have been linked to rare liver injury, especially on an empty stomach.
Red yeast rice	Can contain monacolin K, chemically identical to the statin lovastatin. Don't combine with statins.
Potassium	Dangerous with some blood pressure medicines and kidney disease.

Supplement safety database

Interactions

Supplement warnings

Sources