Supplements and Medicines: What Not to Mix

Written by Dr Mark Wilshere and Dr Marc Crutchley, GMC-registered GPs and founders of Dr Marks. Medically reviewed by Dr Mark Wilshere MBChB MRCGP DCH BSc (Hons).

The short answer: most vitamins are fine alongside most medicines. But a few combinations matter:

Iron, calcium, magnesium and zinc can stop some medicines being absorbed, including levothyroxine, some antibiotics and alendronic acid. Leaving a gap between doses usually solves this.
Vitamin K changes how warfarin works.
St John's wort interacts with many medicines.

If you take a regular prescription, tell your GP or pharmacist about every supplement you use, and ask before starting a new one.

Why supplements and medicines can interact

There are three main ways this happens:

Binding in the gut. Minerals such as iron, calcium, magnesium and zinc can bind to some medicines in the stomach and gut, so less of the medicine is absorbed. This is why timing matters.
Changing how a medicine works. Vitamin K, for example, works against the effect of warfarin.
Changing how the liver handles medicines. Some herbal remedies, St John's wort in particular, speed up the breakdown of many medicines, making them less effective.

It can also work the other way round. Some medicines lower your levels of a nutrient over time.

Common combinations to know about

Medicine	Supplements to be careful with	What the guidance says
Levothyroxine (for an underactive thyroid)	Iron, calcium, antacids, biotin, St John's wort	Take levothyroxine on an empty stomach, 30 to 60 minutes before breakfast. The BNF advises leaving at least 4 hours between levothyroxine and iron or calcium. Biotin can affect thyroid blood test results
Doxycycline and other tetracycline antibiotics	Iron, zinc, magnesium, calcium, antacids	The BNF advises leaving 2 to 3 hours between doses
Ciprofloxacin	Iron, zinc, calcium (including dairy and calcium-fortified drinks)	Leave at least 2 hours between doses
Alendronic acid (for bone health)	Calcium, iron, magnesium, zinc; any other medicine taken by mouth	Take it first thing with plain tap water, then wait at least 30 minutes before food, drink or other medicines
Warfarin	Vitamin K supplements, St John's wort, cranberry products, fish oil, glucosamine	The NHS says not to take St John's wort or vitamin K supplements with warfarin. Speak to your anticoagulant clinic before starting any supplement
Statins (such as atorvastatin, simvastatin)	St John's wort (atorvastatin); grapefruit juice	The NHS atorvastatin page lists St John's wort among the remedies that may not mix, and advises no more than 1 or 2 small glasses of grapefruit juice a day. With simvastatin, the NHS advises avoiding grapefruit juice
Omeprazole and other PPIs	St John's wort	Long-term use can lower magnesium and vitamin B12 levels (see below)
Metformin	None in particular	Long-term use can lower vitamin B12 levels (see below)

These are general guides. Some medicines come in forms with different rules, so always check the leaflet in your medicine pack, or ask your pharmacist.

Warfarin and vitamin K: the one to take most seriously

Warfarin works by blocking the way your body uses vitamin K to make clotting factors. A sudden increase or decrease in vitamin K can change your INR (the blood test that measures how quickly your blood clots).

Supplements: the NHS advises not taking vitamin K supplements while you're on warfarin. Many multivitamins, and most vitamin D3 + K2 products, contain vitamin K, so check the label.
Food: you don't need to avoid green vegetables. The NHS advice is to keep your diet fairly steady and talk to your warfarin clinic before making big changes.
Fish oil: the BNF notes that both warfarin and omega-3 can increase the risk of bleeding. Check with your clinic before starting.

If you take a different anticoagulant, check with your pharmacist before starting any supplement too. Our vitamin K guide explains what the vitamin does.

When medicines lower your nutrient levels

Metformin and vitamin B12. The MHRA says low B12 is a common side effect of metformin, especially at higher doses or over longer periods. It advises B12 tests for anyone with symptoms, and regular checks for people with other risk factors. Read more in metformin and vitamin B12 and our information for people with type 2 diabetes.
PPIs and magnesium. The MHRA has warned that long-term PPI use, usually for more than a year, can cause low magnesium. Symptoms include muscle twitches, tremors, vomiting, tiredness and loss of appetite. The NHS omeprazole page also lists very low vitamin B12 among the possible effects of long-term use. Our article on indigestion medication explains more.

If you take either of these medicines long term, ask your GP whether a blood test is worth doing. Don't just start a supplement, because that can make a deficiency harder to diagnose.

Practical tips for taking supplements safely

Keep a list of every medicine, vitamin and herbal remedy you take, and show it at every GP, pharmacy or hospital appointment.
Space them out. If a mineral-containing supplement and a medicine shouldn't be taken together, take the medicine at its usual time and the supplement at a different meal. Our guide to the best time to take vitamin supplements has more ideas.
Read the whole label. A multivitamin often contains iron, zinc, calcium or vitamin K even when these aren't in the product name.
Tell the person taking blood that you take biotin, which can affect some test results.
Don't stop a prescribed medicine because of a supplement. Speak to your GP or pharmacist first.
Before an operation, tell the surgical team about all supplements and herbal remedies.

When to see your GP or pharmacist

Speak to a pharmacist or your GP:

before starting any supplement if you take warfarin or another anticoagulant, levothyroxine, antiepileptic medicines, immunosuppressants, HIV medicines or chemotherapy
if you're starting a new antibiotic and already take iron, zinc, calcium or magnesium
if you develop unusual bruising or bleeding, or a change in your INR
if you notice new symptoms after starting a supplement

Which Dr Marks products contain these nutrients

So you can check quickly:

Nutrient	Found in
Iron	Men's, Women's and Over 50s Multivitamins
Zinc	Dual Vitamin C, and Men's, Women's and Over 50s Multivitamins
Calcium	Dual Vitamin C (as calcium ascorbate)
Magnesium	Triple Magnesium
Vitamin K	Vitamin D3 + K2, and Men's, Women's and Over 50s Multivitamins
Biotin	Vitamin B Complex, and Men's, Women's and Over 50s Multivitamins
Omega-3 fish oil	Omega 3

Full ingredient lists are on each product page.

Food supplements should not replace a varied, balanced diet and healthy lifestyle.

Frequently asked questions

Can I take a multivitamin with levothyroxine?

Usually yes, but not at the same time. Multivitamins often contain iron and calcium, so the BNF advises leaving at least 4 hours between the two. Many people take levothyroxine on waking and the multivitamin with lunch or dinner.

Can I take vitamins while on antibiotics?

Most antibiotics are fine. With doxycycline and other tetracyclines, or ciprofloxacin, leave a gap of 2 to 3 hours (2 hours for ciprofloxacin) between the antibiotic and any iron, zinc, calcium or magnesium. Ask your pharmacist when you collect your prescription.

Can I take fish oil with blood thinners?

Check with your anticoagulant clinic or pharmacist first. The BNF notes that both warfarin and omega-3 can increase the risk of bleeding.

Can I take magnesium with omeprazole?

There's no known problem with taking them together. Long-term PPI use can lower magnesium, so if you've been on one for more than a year, ask your GP whether you need a blood test.

Can I take vitamin D with my medicines?

For most people, yes. The BNF notes that thiazide diuretics (such as bendroflumethiazide) taken with high-dose vitamin D increase the risk of high calcium levels, so check with your pharmacist if you take both.

Are herbal or over-the-counter supplements always safe with medicines?

No. St John's wort is a herbal remedy, but the NHS lists it as a problem with many medicines, including warfarin, levothyroxine, atorvastatin and omeprazole. The BNF says it reduces the effect of warfarin.

References

NHS. Levothyroxine. https://www.nhs.uk/medicines/levothyroxine/ (accessed 19 October 2026)
NHS. Doxycycline. https://www.nhs.uk/medicines/doxycycline/ (accessed 19 October 2026)
NHS. Ciprofloxacin. https://www.nhs.uk/medicines/ciprofloxacin/ (accessed 19 October 2026)
NHS. Taking alendronic acid with other medicines and herbal supplements. https://www.nhs.uk/medicines/alendronic-acid/taking-alendronic-acid-with-other-medicines-and-herbal-supplements/ (accessed 19 October 2026)
NHS. How and when to take alendronic acid. https://www.nhs.uk/medicines/alendronic-acid/how-and-when-to-take-alendronic-acid/ (accessed 19 October 2026)
NHS. Warfarin. https://www.nhs.uk/medicines/warfarin/ (accessed 19 October 2026)
NHS. Atorvastatin. https://www.nhs.uk/medicines/atorvastatin/ (accessed 19 October 2026)
NHS. Simvastatin. https://www.nhs.uk/medicines/simvastatin/ (accessed 19 October 2026)
NHS. Omeprazole. https://www.nhs.uk/medicines/omeprazole/ (accessed 19 October 2026)
NHS. Metformin. https://www.nhs.uk/medicines/metformin/ (accessed 19 October 2026)
British National Formulary. Interactions: levothyroxine; iron; zinc; magnesium; warfarin; doxycycline; colecalciferol. https://bnf.nice.org.uk/interactions/levothyroxine/ ; https://bnf.nice.org.uk/interactions/iron/ ; https://bnf.nice.org.uk/interactions/zinc/ ; https://bnf.nice.org.uk/interactions/magnesium/ ; https://bnf.nice.org.uk/interactions/warfarin/ ; https://bnf.nice.org.uk/interactions/doxycycline/ ; https://bnf.nice.org.uk/interactions/colecalciferol/ (accessed 19 October 2026)
Medicines and Healthcare products Regulatory Agency (2012). Proton pump inhibitors in long-term use: reports of hypomagnesaemia. https://www.gov.uk/drug-safety-update/proton-pump-inhibitors-in-long-term-use-reports-of-hypomagnesaemia (accessed 19 October 2026)
Medicines and Healthcare products Regulatory Agency (2022). Metformin and reduced vitamin B12 levels: new advice for monitoring patients at risk. https://www.gov.uk/drug-safety-update/metformin-and-reduced-vitamin-b12-levels-new-advice-for-monitoring-patients-at-risk (accessed 19 October 2026)

Food supplements should not replace a varied, balanced diet and healthy lifestyle. Do not exceed the stated daily dose. Keep out of reach of children. If you are pregnant, breastfeeding, taking any medication or under medical supervision, speak to your GP or pharmacist before taking supplements. This article is general information and is not a substitute for personal medical advice. Dr Marks is an independent company and is not affiliated with or endorsed by the NHS.