Supplements and medicines, the interactions to know

How supplements can clash with medicines, the pairs NHS and MHRA advice names, a safety checklist and where each of our ingredient guides covers medicines.

Written by
Evan J.
Last updated
8 min read
Cut-paper illustration of a medicine leaflet, a blister pack and a jar of herbal capsules with a sprig

Overview

Supplements can interact with medicines by blocking absorption, changing how well a medicine works or adding to its effects. NHS and MHRA advice names St John's wort with the pill and several medicines, and calcium, iron or zinc with some antibiotics and thyroid and bone medicines. Tell your pharmacist about every supplement you take.

Key points

  • Supplements can block a medicine's absorption, change how well it works or add to its effects.
  • The MHRA says St John's wort can reduce the effect of hormonal contraception, including implants.
  • NHS pages name calcium, iron, magnesium or zinc with some antibiotics, thyroid and bone medicines.
  • Herbal remedies and supplements generally aren't tested for their effect on medicines, the NHS says.
  • UK food supplement labels must show the amount of each active substance, so take the label to your pharmacist.

Here's a common belief worth testing. Herbal remedies are natural, so they can't clash with your medicines. The UK's medicines watchdog, the MHRA, has found that they can. Reports sent to its Yellow Card scheme first showed that St John's wort clashes with the pill and with some epilepsy medicines [1].

A supplement can stop a medicine getting into your body, change how well it works or add to its effects. This guide covers the supplement and medicine interactions that NHS and MHRA advice names. It also shows how to check your own, and where each of our ingredient guides covers medicines.

How do supplements interact with medicines?

There are three main ways, and NHS and MHRA advice gives an example of each.

  • Getting the medicine in. The calcium in milk and in drinks with added calcium can stop your body taking in all of the antibiotic ciprofloxacin. So the NHS says to leave at least 2 hours between them [2].
  • How well it works. St John's wort can make the pill and other hormone methods of birth control work less well, including implants [3].
  • Adding to an effect. The NHS says not to take herbal remedies that make you sleepy while you take melatonin [4].

For many pairs, no one has checked. On its pages for melatonin and alendronic acid, the NHS says there isn't enough data to call other herbal remedies and supplements safe with them. That's because they aren't usually tested for their effect on other medicines [4][5].

Which supplements interact with common medicines?

These are the pairs that NHS medicine pages and MHRA advice name. It isn't a full list.

Interactions named in NHS and MHRA advice
Supplement Medicine What the advice says
St John's wort The pill, implant and other hormone birth control Don't take it if you rely on them to prevent pregnancy [3]. You may need to use another method [6]
St John's wort Sertraline and warfarin Don't take it with either [7][8]
St John's wort Atorvastatin, amlodipine and levothyroxine Listed as not mixing well, so tell your GP [9][10][11]
Vitamin K Warfarin Don't take vitamin K supplements while on warfarin [8]
Calcium and iron Levothyroxine Listed as not mixing well [11]
Biotin (vitamin B7) Levothyroxine It can affect your thyroid tests [11]
Iron and zinc Doxycycline Listed as not mixing well [12]
Calcium, iron, magnesium and zinc Alendronic acid Tell your GP if you take them [5]
Herbal remedies that make you sleepy Melatonin Don't take them together [4]

If your pair isn't here, that doesn't make it safe. Check the leaflet in the box and ask your pharmacist.

What about ashwagandha, lion's mane and magnesium?

We found no NHS page on ashwagandha or lion's mane. Each of our ingredient guides has a section on medicines, with its own sources.

You can also read the studies behind each ingredient in our Ingredient Atlas.

How to take supplements safely with medicines

Use this checklist when you start a new medicine or a new supplement.

  1. Tell your GP or pharmacist about every supplement. NHS medicine pages ask you to mention any herbal remedies, vitamins or supplements you take [8][12].
  2. Read the leaflet. It lists what may not mix with your medicine. You can also find it online, on the electronic medicines compendium [8].
  3. Bring the label. UK supplement labels must show how much of each active ingredient is in the product. That's what your pharmacist needs to check it [13].
  4. Ask about timing. Some pairs need a gap of a few hours rather than a full stop [2].
  5. Take extra care with warfarin. The NHS says to speak to your doctor before you start any new medicine while you take it [8].
  6. Report side effects. If you think a herbal medicine has caused a reaction, tell your GP or pharmacist, or report it to the Yellow Card scheme yourself [1].

If you've just started a new prescription, a pharmacist can help through the NHS New Medicine Service [9].

How are supplements and herbal medicines regulated?

In the UK, food supplements are covered by food law. They're not medicines, so they aren't meant to treat or prevent disease. The seller must make sure they're safe [13]. Our guide to reading supplement labels shows what the label must include.

Herbal medicines are different. Licensed ones carry a THR number and mark, which stands for Traditional Herbal Registration, or a PL number for a product licence. These meet set standards for quality and safety, and come with a leaflet [1].

No warning on the pack doesn't mean no risk. The MHRA found that some unlicensed St John's wort products sold in the UK or online had no warning about other medicines. It said that doesn't mean they don't interact [3].

Where Nooroots fits

Our products are food supplements. Each product page lists what's in a daily serving, so you can show it to your pharmacist. Triple Magnesium gives 301 mg of elemental magnesium in three capsules. Ashwagandha + Lion's Mane gives 120 mg of KSM-66 ashwagandha root extract and 500 mg of lion's mane fruiting body extract in two capsules.

If you take a medicine, check the guides above and ask your pharmacist before you start either one.

Frequently asked questions

Is there a herbal drug interaction checker?

Your pharmacist can check for you. Each NHS medicine page also lists herbal remedies and supplements that may not mix well with it, and so does the leaflet in the box [11][12].

Does St John's wort stop the pill working?

It can. The MHRA says products containing St John's wort can reduce the effect of hormonal contraception, including the pill and implants, so don't take it if you rely on them [3].

What is St John's wort?

It's a herbal remedy some people take for low mood or depression. The NHS says doctors don't recommend it, partly because its strength varies between brands and batches, and it can cause serious problems with some medicines [3][14].

Can I take vitamins or minerals with antibiotics?

It depends on the antibiotic. The NHS lists supplements with iron or zinc as not mixing well with doxycycline, and advises a 2-hour gap between ciprofloxacin and dairy or calcium-added drinks [2][12].

Can you take magnesium with blood pressure tablets?

Often, yes. The NHS page for amlodipine, a common blood pressure tablet, doesn't list magnesium among the things that may not mix well with it, though it asks you to tell your doctor about any supplements [10]. Our guide to when to take magnesium checks magnesium with other blood pressure tablets and the gap to leave.

Can you take turmeric with statins?

Ask your pharmacist first. The NHS page for atorvastatin doesn't name turmeric, but it asks you to tell your doctor about any herbal remedies or supplements you take [9].

Can you take ashwagandha with levothyroxine?

Ask your GP or pharmacist first. Our ashwagandha side effects guide covers thyroid medicines and the others to check.

Read more about what a supplement label must show in our guide to reading supplement labels.

References

Every source here is listed so you can read it yourself. This guide is for general information and doesn't replace advice from your GP or pharmacist. We check it again at least every six months.

  1. Medicines and Healthcare products Regulatory Agency. Herbal and homeopathic medicines: reminder to be vigilant for suspected adverse reactions and to report them to the Yellow Card scheme. Drug Safety Update, 7 July 2021. Accessed 31 October 2026. gov.uk
  2. NHS. Ciprofloxacin. Page last reviewed 9 October 2026. Accessed 31 October 2026. nhs.uk/medicines/ciprofloxacin
  3. Medicines and Healthcare products Regulatory Agency. St John's wort: interaction with hormonal contraceptives, including implants. Drug Safety Update, March 2014. Accessed 31 October 2026. gov.uk
  4. NHS. Taking melatonin with other medicines and herbal supplements. Page last reviewed 13 February 2023. Accessed 31 October 2026. nhs.uk/medicines/melatonin
  5. NHS. Taking alendronic acid with other medicines and herbal supplements. Page last reviewed 7 May 2025. Accessed 31 October 2026. nhs.uk/medicines/alendronic-acid
  6. NHS. How to take the combined pill. Page last reviewed 15 February 2024. Accessed 31 October 2026. nhs.uk/contraception
  7. NHS. Sertraline. Page last reviewed 8 September 2025. Accessed 31 October 2026. nhs.uk/medicines/sertraline
  8. NHS. Warfarin. Page last reviewed 24 February 2026. Accessed 31 October 2026. nhs.uk/medicines/warfarin
  9. NHS. Atorvastatin. Page last reviewed 23 June 2026. Accessed 31 October 2026. nhs.uk/medicines/atorvastatin
  10. NHS. Amlodipine. Page last reviewed 9 June 2026. Accessed 31 October 2026. nhs.uk/medicines/amlodipine
  11. NHS. Levothyroxine. Page last reviewed 6 July 2026. Accessed 31 October 2026. nhs.uk/medicines/levothyroxine
  12. NHS. Doxycycline. Page last reviewed 5 September 2025. Accessed 31 October 2026. nhs.uk/medicines/doxycycline
  13. Food Standards Agency. Food supplements: guidance for businesses. Published 2 July 2018. Accessed 31 October 2026. gov.uk
  14. NHS. Treatment: depression in adults. Page last reviewed 5 July 2023. Accessed 31 October 2026. nhs.uk

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