Drug and supplement interactions, and how to stay safe

How interactions happen

A supplement can change how a medicine is absorbed, broken down or cleared from the body, so that you get too much or too little of the medicine you need [1]. Others add to a medicine’s effect. Warfarin, ginkgo, aspirin and vitamin E can each thin the blood, and taking them together may raise the chance of internal bleeding or stroke [1].

Minerals can also bind to medicines in the gut. The advice for magnesium hydroxide, an antacid and laxative, is to take other medicines at least 2 hours before or after it [2]. The effect can run the other way too, and long-term use of acid-reducing proton pump inhibitors can lower magnesium in the blood [3].

A product described as natural or herbal can interact as strongly as a prescribed medicine, and children, people who are pregnant or breastfeeding and anyone facing surgery need particular care [1].

St John’s wort, the clearest example

St John’s wort is a herbal remedy traditionally used for slightly low mood and mild anxiety [4]. A review by medicines regulators in the UK and Sweden found clinically significant interactions with warfarin, ciclosporin, HIV medicines, theophylline, digoxin and oral contraceptives, lowering the level or effect of each, because the herb speeds up the liver enzymes and transporters that clear them [5]. How strong the effect is cannot be predicted, because the content of products varies [5].

The MHRA has received 19 reports of suspected interactions with hormonal contraceptives through the Yellow Card scheme since 2000, and 15 of them involved unplanned pregnancies [4]. Its advice is that women using hormonal contraception to prevent pregnancy should not take St John’s wort, and it adds that some unlicensed products sold online carry no interaction warning at all [4]. People with epilepsy are advised to avoid it with any antiepileptic medicine [6].

Taken with antidepressants or other medicines that affect serotonin, St John’s wort can also cause serotonin-related side effects, which can be serious [7].

Warfarin and other blood thinners

Warfarin is the medicine where small changes matter most. Products that may interact with it include coenzyme Q10, echinacea, garlic, ginkgo, ginseng, goldenseal and St John’s wort [8]. Vitamin K in food also affects how warfarin works, so the advice is to keep the amount you eat steady from week to week rather than avoid it [8].

If you take any blood thinner, start a new supplement only after checking with the doctor or pharmacist who manages it. A dose change may need a blood test to confirm that your level is still right.

Before surgery or a procedure

Some supplements interact with the medicines used around an operation. Your doctor may ask you to stop supplements two or three weeks beforehand, to avoid changes in heart rate, blood pressure or bleeding [1]. Tell the surgical team about everything you take as soon as a date is set.

What to bring to your doctor or pharmacist

A complete list is the most useful thing you can bring, and it is worth carrying a copy with you [8].

• Every medicine, vitamin, mineral, herbal remedy and supplement you take, with the dose and how often [1].
• The packs, or clear photos of the labels, including anything bought online or abroad [1].
• Any change in your health, such as pregnancy, breastfeeding, a recent illness or planned surgery [1].
• For herbal medicines, whether the pack shows a Traditional Herbal Registration mark and number or a product licence number, which shows it is licensed by the MHRA [9].
• Anything new you plan to start, so it can be checked before the first dose.

Reporting a reaction

If you think a medicine, supplement or herbal remedy has caused a side effect, tell your doctor or pharmacist, or report it yourself through the MHRA Yellow Card scheme on its website or app [10]. Yellow Card reports are how the interactions between St John’s wort and contraceptives and antiepileptic medicines first came to light, yet in a study in Wales fewer than 1 in 3 people knew they could report a reaction to a herbal medicine [9].

If you have bleeding that will not stop, vomit blood or something that looks like coffee grounds, have a seizure or develop chest pain, call 999 in the UK or the local emergency number.

How The Wellness can help

A medication review in a GP appointment gives you time to go through every medicine and supplement you take with a doctor, check them for interactions and agree what to keep, change or stop. If a change needs a prescription or a blood test, the doctor arranges it and explains the result with you.

Every doctor is GMC registered. Our clinics are on Baker Street in Marylebone and Eccleston Street in Belgravia, and a doctor can come to your home or hotel in London if that is easier.