Supplement and Drug Interactions: Which Should Not Be Taken Together?

Short Answer

Calcium and iron reduce the absorption of thyroid medication; the TEMD 2025 thyroid guideline recommends at least 4 hours between them. Vitamin K changes the effect of warfarin; St. John's wort can weaken many drugs, including birth control pills and blood thinners. Red yeast rice causes problems with statins, and potassium with some blood pressure drugs. Tell your physician and pharmacist about every supplement; do not change your medication on your own.

Summary table: the most important interactions

The table below summarizes common situations. Time intervals are taken from the TEMD guidelines and from the US National Library of Medicine's drug information (MedlinePlus). What applies to you is the package leaflet of your own medication and your pharmacist's advice.

Supplement	Drug	What happens?	What to do?
Calcium, iron	Levothyroxine (thyroid hormone)	The drug is absorbed less	At least 4 hours apart
Calcium, magnesium, iron, zinc	Quinolone antibiotics (e.g., ciprofloxacin)	The antibiotic is absorbed less	Follow the time interval in the package leaflet
Calcium, magnesium, iron	Oral bisphosphonate (osteoporosis drug)	The drug is absorbed much less	On an empty stomach, with water only; do not lie down afterward
Vitamin K	Warfarin	The blood's clotting balance changes	Do not change intake suddenly
Red yeast rice	Statin	You end up taking a statin on top of a statin	Not used together
St. John's wort	Birth control pill, antidepressant, warfarin, rivaroxaban	The drug's effect decreases or side effects increase	Do not use without consulting your physician
Potassium, potassium-containing salt	ACE inhibitor, ARB, potassium-sparing diuretic	Blood potassium may rise	Consult your physician
Biotin (high dose)	Thyroid and some hormone tests	The result may be wrong	Tell the laboratory

Minerals: thyroid medication, antibiotics and bone drugs

Calcium, iron, magnesium and zinc can bind to some drugs in the intestine. This binding reduces the drug's passage into the blood. As a result, the drug may not have the expected effect.

• Levothyroxine (thyroid hormone): The TEMD 2025 thyroid guideline recommends at least 4 hours between the drug and ferrous sulfate or calcium carbonate. According to the guideline, when a new drug that affects absorption is started, TSH is measured 6–8 weeks later.
• Quinolone antibiotics: According to MedlinePlus ciprofloxacin information, supplements and antacids containing calcium, magnesium, iron or zinc interact with this drug. These products should be taken at least 6 hours before or 2 hours after ciprofloxacin. The interval is not the same for every antibiotic. If you use another quinolone or a tetracycline-group antibiotic, read the package leaflet and ask your pharmacist.
• Bisphosphonates: According to the TEMD 2025 osteoporosis guideline, oral bisphosphonates are swallowed in the morning on an empty stomach with a glass of water. Afterward, you stay upright for 30 minutes with alendronate and risedronate, and 60 minutes with ibandronate; do not lie on your back. Drinks other than water, food, and drugs containing calcium, magnesium, iron or aluminum markedly reduce absorption. Ask your pharmacist what time to take your calcium and vitamin D supplement.

Multivitamins and "bone support" products may also contain these minerals. Check the ingredient list on the box and show it to your pharmacist. Iron's interactions with other drugs such as levodopa are covered in our iron supplement guide.

Blood thinners and cholesterol drugs

Vitamin K and warfarin: According to NIH ODS, people using warfarin and similar drugs should keep their vitamin K intake steady. Sudden changes in vitamin K from food or supplements can increase or decrease the drug's effect. You do not need to give up green leafy vegetables completely; what matters is eating them regularly and in similar amounts. The vitamin K rule is different for newer blood thinners such as apixaban, rivaroxaban, edoxaban or dabigatran; details are in the vitamin K and warfarin guide. However, these drugs are also affected by herbs. The US-approved rivaroxaban product information says the drug should not be used together with St. John's wort; St. John's wort lowers the drug's level in the blood. If you use a newer blood thinner, do not start an herbal product without consulting your physician.

Red yeast rice and statins: According to European Union Regulation No 2022/860, monacolin K in red yeast rice is the same substance as lovastatin. So a person on a statin who takes this product ends up taking a second statin. The EU banned products containing 3 mg or more of monacolins per daily portion. The label must state that people using cholesterol medication should not use this product. The same warning also covers pregnant women, breastfeeding women, people under 18 and people over 70.

Metformin, stomach protectors and B12

The NICE NG239 (2024) guideline lists some drugs among the risk factors for B12 deficiency. These are metformin, proton pump inhibitors (stomach protectors) and H2 receptor blockers. Colchicine, phenobarbital, pregabalin, primidone and topiramate are also on the list. If you use these drugs long term, discuss B12 testing with your physician.

St. John's wort and other herbal products

St. John's wort is a plant that can interact with many drugs. According to the US National Center for Complementary and Integrative Health (NCCIH), it can reduce the effect of the following drugs:

• Birth control pills
• Some antidepressants such as amitriptyline and bupropion
• Warfarin
• Cyclosporine, used after organ transplantation
• Some seizure (epilepsy) drugs such as phenytoin and carbamazepine
• Some statins such as simvastatin
• Some heart drugs such as digoxin and ivabradine
• Some HIV drugs such as indinavir and nevirapine
• Some cancer drugs such as irinotecan, imatinib and docetaxel

The product information of newer blood thinners such as rivaroxaban also asks that use with St. John's wort be avoided. According to NCCIH, St. John's wort can cause serious side effects together with antidepressants that affect serotonin. The list is only examples. Do not consider tea, capsule or drop forms safe; if you use a drug not on the list, ask your physician as well. Supplements that can increase bleeding risk and what to do before surgery are covered in our herbal supplement risks guide.

Potassium and blood pressure drugs

According to NIH ODS, ACE inhibitors and ARB-group blood pressure drugs reduce potassium excretion in urine. Potassium-sparing diuretics can also raise potassium. Using a potassium supplement or potassium-containing salt with these drugs can lead to high blood potassium. Thiazide and loop diuretics, on the other hand, lower potassium. Details are in our low potassium guide.

Biotin and test results

Biotin interacts not with a drug but with a laboratory test. According to the TEMD 2025 thyroid guideline, high-dose biotin, such as 5–10 mg per day, gives false results with some methods. TSH may come out low and free T4 high, and this picture may be mistaken for hyperthyroidism. The guideline recommends stopping biotin at least 2 days before thyroid tests. The time needed for other tests varies by method; ask your laboratory. For details, see the biotin and lab tests guide.

What does the evidence say?

Most interaction information comes from drug labels, absorption studies and case reports. The size of the effect varies from person to person. But some interactions are strongly documented:

• Red yeast rice: As reported in the EU regulation, in 2018 EFSA reported side effects of monacolins such as muscle breakdown (rhabdomyolysis) and liver inflammation. There were isolated serious cases even at intakes as low as 3 mg per day.
• B12 risk: NICE NG239 recognizes metformin and stomach protectors as risk factors for B12 deficiency.
• St. John's wort: NCCIH emphasizes that interactions can weaken the effect of life-saving drugs.

Those who need to be most careful are people taking many medications, older adults, people with kidney or liver disease, and pregnant women.

In Türkiye

• Thyroid guideline: The TEMD 2025 thyroid guideline lists calcium carbonate, ferrous sulfate, omeprazole and lansoprazole among the agents that reduce levothyroxine absorption. The same guideline also states that high-dose biotin interferes with thyroid tests.
• Osteoporosis guideline: The TEMD 2025 osteoporosis guideline states that drugs containing calcium, magnesium and iron reduce the absorption of oral bisphosphonates.
• Label: We explained how to read the warnings on the supplement box in our supplement label guide.
• Practical step: Every time you get a prescription, also tell your pharmacist which supplements you use. The pharmacist helps you plan the time interval between your medication and your supplement.

Which doctor should you see?

• First contact: The physician who prescribed your medication or your family physician; your pharmacist for an initial check.
• If you use thyroid medication: Endocrinology or Internal Medicine; if your TSH result has changed, mention your supplements.
• If you use a blood thinner: Cardiology or the relevant clinic that prescribed the drug; the unit that monitors your INR.
• Older adults taking many medications: Internal Medicine or Geriatrics.
• Bring with you: The boxes of all medications, vitamins, minerals and herbal products, and your latest test results.

When should you seek emergency care?

• If you have black stools, bloody urine, vomiting blood or bleeding that does not stop while on a blood thinner, go to the emergency department without waiting. If there is fainting, vomiting blood or heavy bleeding, call 112 (the emergency number in Türkiye).
• If you have a sudden severe headache, speech disturbance or weakness on one side while on a blood thinner, call 112.
• If you have palpitations, fainting or sudden muscle weakness while using a potassium supplement or potassium-containing salt, call 112.
• If you have restlessness, tremor, high fever, muscle spasms or a fast heartbeat while using St. John's wort together with an antidepressant, call 112.
• If a child has swallowed a product containing iron, call 112 without waiting for symptoms.
• If a child has swallowed another supplement or drug, call 114, the National Poison Information Center. If there is drowsiness, vomiting, difficulty breathing or a seizure, call 112.

Related guides

• Vitamin and Mineral Guide
• Drug interaction checker
• Vitamin K and warfarin
• Does biotin interfere with lab results?
• B12 deficiency while using metformin
• Can omega-3 be used with a blood thinner?
• Multiple medication use in older adults
• How to read a supplement label

What changed? (October 2026)

• New guide: B12 risks are given according to NICE NG239 (2024), and thyroid medication and biotin information according to the TEMD 2025 guideline; a St. John's wort–newer blood thinner warning was added.
• Label warnings of EU Regulation 2022/860 for red yeast rice were added.
• The TEMD 2025 osteoporosis guideline was added for osteoporosis drugs (bisphosphonates); the rule of staying upright after taking the drug was also included.
• Minerals that interact with quinolone antibiotics were expanded to calcium, magnesium, iron and zinc.

Sources

• Türkiye Endokrinoloji ve Metabolizma Derneği. Tiroid Hastalıkları Tanı ve Tedavi Kılavuzu. 8th edition, April 2025. https://file.temd.org.tr/Uploads/publications/guides/documents/tiroid2025_11042025.pdf (accessed October 4, 2026)
• Turkish Ministry of Health (T.C. Sağlık Bakanlığı). 114 UZEM (Ulusal Zehir Danışma Merkezi) call center. 2026. https://www.saglik.gov.tr/TR,11518/114-uzem.html (accessed October 4, 2026)
• Türkiye Endokrinoloji ve Metabolizma Derneği. Osteoporoz ve Metabolik Kemik Hastalıkları Tanı ve Tedavi Kılavuzu. 17th edition, April 2025. https://file.temd.org.tr/Uploads/publications/guides/documents/OSTEPOROZ_nisan2025.pdf (accessed October 4, 2026)
• US National Library of Medicine, DailyMed. Rivaroxaban tablet, FDA-approved product information, Sections 5.6 and 7.3. Updated September 4, 2026. https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=481b7802-5093-43e7-bbd9-1532197eb6e6 (accessed October 4, 2026)
• US National Library of Medicine, MedlinePlus. Ciprofloxacin: drug information. Updated August 15, 2026. https://medlineplus.gov/druginfo/meds/a688016.html (accessed October 4, 2026)
• NIH Office of Dietary Supplements. Vitamin K: Fact Sheet for Health Professionals. Updated March 29, 2021. https://ods.od.nih.gov/factsheets/VitaminK-HealthProfessional/ (accessed October 4, 2026)
• NIH Office of Dietary Supplements. Potassium: Fact Sheet for Health Professionals. Updated June 2, 2022. https://ods.od.nih.gov/factsheets/Potassium-HealthProfessional/ (accessed October 4, 2026)
• NICE. Vitamin B12 deficiency in over 16s: diagnosis and management (NG239). March 2024. https://www.nice.org.uk/guidance/ng239/chapter/Recommendations (accessed October 4, 2026)
• NCCIH. St. John's Wort. Updated May 2025. https://www.nccih.nih.gov/health/st-johns-wort (accessed October 4, 2026)
• European Commission. Commission Regulation (EU) 2022/860 of 1 June 2022: monacolins from red yeast rice. https://eur-lex.europa.eu/eli/reg/2022/860/oj/eng (accessed October 4, 2026)