saudigastro.com/patients/supplement-medicine-interactions
This article is educational and does not replace your doctor's advice. Seek urgent care for any red-flag symptoms.
Drug class
Dietary supplements, herbal products and food that interact with medicines
Examples
St John’s wort, grapefruit, calcium and iron tablets, vitamin K, fish oil and vitamin E, antacids, fibre products, and herbal mixtures. Brand names are not listed.
How it works
Interactions happen in three main ways. (1) Absorption: minerals, antacids and fibre can bind a medicine in the gut so less enters the blood. (2) Metabolism: some herbs and foods speed up or slow the liver and gut enzymes (such as CYP3A4) and the transporter P-glycoprotein that clear medicines, so drug levels fall or rise. (3) Effect: two substances with the same action (for example blood thinning) add together. The risk is highest for medicines with a narrow safety margin: warfarin, tacrolimus, ciclosporin, digoxin, levothyroxine and some anti-seizure and HIV drugs. Interaction knowledge comes from pharmacology studies, case reports and drug-labelling databases; evidence for many herbal interactions is limited, so caution is advised. The NIH National Center for Complementary and Integrative Health (NCCIH) and the Office of Dietary Supplements recommend telling all your health professionals about every product you use.
What it treats
- St John’s wort (a herbal “mood” product) is one of the strongest herbal interactors: it lowers blood levels of many drugs, including tacrolimus and ciclosporin (risk of transplant rejection), warfarin, hormonal contraceptives, some HIV drugs and many others. Avoid it unless your doctor and pharmacist have checked
- Grapefruit and pomelo (and Seville orange): raise levels of some drugs by blocking gut CYP3A4 (for example tacrolimus, ciclosporin, some statins and calcium-channel blockers). Ask before eating or drinking them regularly
- Calcium, iron, magnesium, aluminium (antacids), zinc and multivitamins with minerals: bind levothyroxine (thyroid hormone) and some antibiotics (tetracyclines, ciprofloxacin and similar quinolones), reducing absorption; separate them by several hours as your pharmacist advises
- Vitamin K (in supplements, and in green leafy vegetables) opposes warfarin: keep your intake steady and do not start vitamin K supplements without your clinic’s advice. Newer anticoagulants are not affected by vitamin K
- Fish oil, vitamin E, ginkgo, garlic, ginger and turmeric supplements: can add to bleeding with warfarin, other anticoagulants, aspirin and NSAIDs; relevant before endoscopy and surgery
- Acid-reducing medicines (PPIs and H2 blockers) over the long term can lower absorption of vitamin B12, iron and magnesium (and can contribute to low magnesium, which also lowers calcium); they also reduce absorption of some medicines that need acid (for example certain antifungals and some HIV drugs). Your doctor may check levels
- Fibre products (psyllium and similar) can slow absorption of other medicines; take medicines about 2 hours before or after, as advised
- Herbal products with immunosuppressants (tacrolimus, ciclosporin, azathioprine, mycophenolate, methotrexate) and biologics: echinacea and other “immune boosters” may counter immunosuppression in theory; herbs affecting liver enzymes can change tacrolimus levels; herbal liver toxicity adds to methotrexate or azathioprine risk. Evidence is mostly from case reports and mechanisms (LIMITED); the practical rule is to ask your transplant, IBD or liver team before taking any product
How to take it
Tell-your-pharmacist checklist: (1) keep a written list of EVERYTHING you take: prescribed medicines, over-the-counter medicines, vitamins, minerals, herbs, teas, protein powders and traditional remedies, with product names, strengths and reasons; (2) carry the list or photos of the labels to every appointment, including dental and endoscopy visits; (3) use one pharmacy where possible; (4) ask “can I take this with my medicines, and how far apart?” before adding anything new; (5) do not stop a prescribed medicine to make room for a supplement; (6) tell the team about grapefruit, large amounts of green vegetables if on warfarin, and alcohol; (7) report new symptoms after starting a product; (8) follow the label and your doctor’s or pharmacist’s advice on any spacing. BEFORE ENDOSCOPY OR SURGERY: ask your team which supplements to pause. They commonly ask people to stop iron tablets (they darken stool and can make the bowel preparation less clear) about a week before a colonoscopy, and to pause some herbal and omega-3 or vitamin E products before procedures with biopsy or polyp removal. Do not stop prescribed blood thinners or heart medicines unless the team tells you exactly how (see our article on blood thinners before endoscopy). Practices differ between hospitals, so follow your own unit’s written instructions.
Common side effects
- Many interactions cause no symptoms at all, which is why asking is important
- A medicine that seems to “stop working” (for example thyroid symptoms returning, or a transplant drug level dropping)
- Stronger-than-expected medicine effects, such as dizziness, drowsiness or easy bruising
- Stomach upset, constipation or diarrhoea from the supplement itself
Serious side effects — call your doctor
Unusual bleeding: black or bloody stools, vomiting blood, nosebleeds that will not stop, large bruises, blood in urine
Possible dangerous interaction with a blood thinner (for example warfarin with a herbal or fish-oil product). Go to the emergency department.
Fever, flu-like illness, new tiredness or reduced urine in a transplant patient, or a fall in tacrolimus level after starting a herb
Possible rejection or drug toxicity from an interaction. Contact your transplant team the same day.
Palpitations, fainting, chest pain, or severe weakness while taking a new product with heart medicines
Could be a heart-rhythm effect. Call emergency services or go to the emergency department.
Yellow eyes or skin, dark urine, severe itching or upper right belly pain after starting a herb or supplement
Possible liver injury. Stop the product and see a doctor urgently.
Rash, swelling of the lips, face or throat, wheeze or trouble breathing
Could be a serious allergic reaction. Call emergency services now.
Return of symptoms of an under-treated condition: hypothyroid symptoms (tiredness, cold intolerance), seizures, or recurrent infection
A supplement may be reducing the medicine’s effect. See your doctor promptly (urgently for seizures).
Interactions
The table of examples in the section above is a summary and not complete. Antacids and calcium: separate from levothyroxine and quinolone or tetracycline antibiotics (a gap of several hours is usually advised; ask your pharmacist for the exact timing for your medicine). Iron and calcium also compete for absorption. Warfarin: stable vitamin K intake; check any new herb, fish oil, vitamin E or cranberry product with the anticoagulant clinic. Fibre: space from other medicines. PPIs: long-term use can lower B12, iron and magnesium. Immunosuppressants (tacrolimus, ciclosporin, azathioprine, mycophenolate, methotrexate) and biologics: avoid St John’s wort, grapefruit, and unregulated “immune-boosting” or liver-affecting herbal products unless your specialist agrees. Sources for these statements include medicine product information, the NIH NCCIH and Office of Dietary Supplements, and LiverTox. Interaction data for many herbal products is limited, so “not known to interact” is not the same as “safe”.
Monitoring
Your doctor or pharmacist may adjust monitoring when you add or stop a product: INR for warfarin, drug levels for tacrolimus or ciclosporin, thyroid tests (TSH) for levothyroxine, B12, iron, magnesium and calcium for people on long-term PPIs, and liver tests if a herb is suspected. Review your list at every visit and after any hospital stay.
When and how to stop
Stop a supplement or herb and call your doctor if you notice any red-flag symptom or if a monitored result (INR, drug level, TSH) changes unexpectedly after you started it. Do not stop prescribed medicines on your own because of a possible interaction; call your pharmacist or doctor first.
Pregnancy & breastfeeding
Many supplements and herbs are not studied in pregnancy and breastfeeding, and interactions with pregnancy medicines are possible. Review every product with your doctor, midwife or pharmacist; prenatal vitamins containing iron and calcium also need spacing from thyroid tablets.
In the Saudi context
In Saudi Arabia many people take herbal remedies, black seed, honey blends, vitamins and imported supplements alongside prescribed medicines, often from herbal shops (attar), social media or relatives, and do not mention them to the doctor. Ask your clinic whether they accept a photo of your supplement list. Remember that grapefruit and pomelo are on sale in local markets and that large servings of green leafy vegetables (for example jarjeer, molokhia and spinach) matter for warfarin users. During Ramadan, tablet timing changes (iftar and suhoor), so ask your pharmacist to redraw the spacing between supplements and medicines such as levothyroxine, iron and antibiotics.
Questions to ask your doctor
Save this list or print it before your appointment — doctors appreciate prepared patients.
- Can you check my full list of medicines and supplements for interactions?
- How far apart should I take my thyroid tablet, iron, calcium and antibiotics?
- Which supplements must I pause before my endoscopy, and for how long?
- I take a transplant or IBD immunosuppressant: is any herb or supplement safe for me?
- Since I take a PPI long term, should I have B12, iron or magnesium checked?
Trusted resources
Was this guide helpful?
Status: Approved for publication by SGA; individual clinician review is ongoing · Published 24/04/2026
For general education only; it does not replace your doctor’s advice. Individual clinician review of this guide is ongoing. SGA clinician? Review this guide