Supplements and vitamins: when can they interfere with your medicines?

"Natural" does not mean harmless: some supplements and vitamins change how your medicines work, so it is worth checking first.

Supplements and vitamins can interfere with your medicines when they reduce absorption, change how the body clears them or add a similar effect. It happens with very common products, such as St John's wort, ginkgo or preparations with calcium and iron, and mainly affects medicines such as levothyroxine, anticoagulants or some antibiotics.

If you manage your own medication, knowing which combinations to watch helps you decide more safely. Here are the best known ones and what to do before starting any supplement.

Why can a supplement interfere with a medicine?

Because many supplements contain active substances that the body processes just like a drug. They can reduce how well a medicine is absorbed in the gut, speed up or slow down its breakdown in the liver, or add a similar effect, for example on blood clotting.

The fact that a product is sold without a prescription, in a herbalist's shop or online, or is marketed as "natural", does not mean it cannot interact. That is why your doctor and pharmacist need to know everything you take, not just what you have been prescribed.

Which supplements cause the most problems?

Some interactions are well documented and worth knowing. This table summarises those most often cited by official sources; it is not a complete list.

If you take anticoagulants, any new supplement deserves a conversation first: MedlinePlus advises talking to your healthcare professional before taking vitamins, supplements or herbal products if you take warfarin. There is more on common combinations in interactions between medicines.

What about ordinary vitamins and minerals?

They can interfere too, especially minerals. Calcium, iron or magnesium bind to some medicines in the gut and reduce their absorption, so spacing them a few hours apart is often enough. The problem is that many multivitamins contain these minerals without you realising.

• Levothyroxine: the Spanish summary of product characteristics asks for at least 2 hours between it and calcium or iron salts; some international sources, such as MedlinePlus, say 4 hours. Ask your pharmacist which applies to you.
• Bisphosphonates for osteoporosis: they are taken on an empty stomach and apart from calcium and other food, as their leaflet explains.
• Vitamin K: in people taking acenocoumarol or warfarin, sudden changes in intake, including supplements that contain it, can upset control. Consistency is what matters.
• High doses: do not exceed the dose on the pack; more is not better and it increases the risk of unwanted effects.

What should you do before starting a supplement?

1. Ask yourself why you want it and mention it to your doctor: sometimes the symptom has another cause worth checking.
2. Take the pack to the pharmacy and ask whether it is compatible with your medicines. The pharmacist can check for known interactions.
3. Add it to your medication list, with the dose and time. We explain how in your personal medication list.
4. Mention that you take it before an operation or a blood test, or when you are prescribed something new.
5. If you already take it and are worried about an interaction, do not stop it suddenly without advice: in the case of St John's wort, the AEMPS warned that stopping it can raise the levels of other medicines.

A practical example: Javier and his mood supplement

Let us take the case of Javier, 52, a sales representative, who takes levothyroxine for an underactive thyroid and an anticoagulant for atrial fibrillation. A very stressful period leads him to buy a "natural" mood product containing St John's wort online and, on a friend's recommendation, a multivitamin with calcium and iron that he takes with his levothyroxine in the morning.

At the pharmacy, when collecting his medication, he mentions the product in passing. His pharmacist explains that St John's wort can interact with several anticoagulants and other medicines, and advises him not to take it without speaking to his doctor first. She also suggests spacing the multivitamin from the levothyroxine by the time stated in the summary of product characteristics. Javier sees his doctor, who reviews his case, and updates his medication list with everything he takes.

How can IMA Health help?

When you take several medicines plus a supplement, keeping each dose properly spaced is key. With IMA Health, a specialised pharmacy prepares your medication by day and time of each dose, and personalised follow-up lets you mention changes, such as a new supplement, so the professionals can take them into account. IMA coordinates with the pharmacy and delivers to your home.

Sources

• AEMPS: Information note on Hypericum perforatum (St John's wort) interactions (in Spanish)
• CIMA, AEMPS: Eutirox (levothyroxine sodium) summary of product characteristics (in Spanish)
• Medicines Portal, Junta de Castilla y León: Food and drug interactions (in Spanish)
• NCCIH (National Institutes of Health): Herb-Drug Interactions
• MedlinePlus: Taking warfarin (in Spanish)

This article is for information only and does not replace advice from your doctor or pharmacist.