Guides
Herbal Products and Drug Interactions
"Natural" does not mean a herb cannot interact with medicines. The interactions below are listed in the herbs' EMA monographs. If you take medicines regularly, tell your doctor or pharmacist before starting a herbal product.
Updated: October 15, 2026
The herb with the most interactions: St John's wort
St John's wort induces drug-metabolising liver enzymes (CYP3A4 and others) and lowers the blood levels of many medicines.
- Contraceptive pills: breakthrough bleeding and reduced protection; use an extra method
- Anticoagulants such as warfarin, ciclosporin, tacrolimus, HIV and some cancer drugs: combined use is contraindicated
- Antidepressants such as sertraline or paroxetine: risk of serotonin syndrome
Herbs that may increase bleeding
Herbs that need care if you take anticoagulants or antiplatelet medicines:
- Ginkgo: may change the effect of anticoagulants and antiplatelets; stop 3–4 days before surgery
- Garlic preparations: may prolong bleeding; stop 7 days before surgery
- Willow bark: may increase the effect of coumarin anticoagulants
- Saw palmetto: raised INR reported in a few people on warfarin
Herbs that delay absorption of medicines
Herbs rich in fibre or mucilage can delay the absorption of medicines taken at the same time. Leave at least 30–60 minutes between medicines and psyllium, flaxseed or marshmallow root.
For psyllium the EMA specifically names minerals, vitamin B12, digoxin, coumarins, carbamazepine and lithium; diabetes and thyroid medicines may need dose adjustment.
Other important examples
- Green tea (caffeine): may reduce the effect of sedatives
- Fenugreek: may lower blood sugar; monitor it if you take diabetes medicines
- Ginkgo: not recommended with efavirenz; may raise dabigatran and nifedipine levels
- Garlic: must not be combined with saquinavir/ritonavir
What to do
Tell your doctor and pharmacist about every herbal product you use, and mention it before surgery. On Herbadex, each herb card lists the interactions from its monograph under "Safety and interactions".
Frequently asked questions
Can I take St John's wort with the pill?
Not recommended. According to the EMA it can reduce the effect of hormonal contraception; users should add another method.
Which herbs should I watch on blood thinners?
Per EMA monographs: ginkgo, garlic preparations, willow bark and St John's wort; raised INR has also been reported with saw palmetto.
Can I take psyllium with my medicines?
Not at the same time; leave at least 30–60 minutes because it may delay their absorption.
Sources
- European Union herbal monograph on Althaea officinalis L., radixEMA HMPC · 2015
- Community herbal monograph on Linum usitatissimum L., semen – Revision 1EMA HMPC · 2015
- Community herbal monograph on Plantago ovata Forssk., seminis tegumentum – Revision 1EMA HMPC · 2013
- European Union herbal monograph on Salix [various species including S. purpurea L., S. daphnoides Vill., S. fragilis L.], cortex – Revision 1EMA HMPC · 2017
- European Union herbal monograph on Allium sativum L., bulbusEMA HMPC · 2017
- European Union herbal monograph on Trigonella foenum-graecum L., semen – Revision 1EMA HMPC · 2021
- Community herbal monograph on Camellia sinensis (L.) Kuntze, non fermentatum foliumEMA HMPC · 2013
- European Union herbal monograph on Hypericum perforatum L., herba – Revision 1EMA HMPC · 2022
- European Union herbal monograph on Ginkgo biloba L., foliumEMA HMPC · 2015
- European Union herbal monograph on Serenoa repens (W. Bartram) Small, fructusEMA HMPC · 2015
Related herbs
Fenugreek
T Traditional useTrigonella foenum-graecum
Fenugreek seed is traditionally used before meals for temporary loss of appetite and as a poultice for minor skin inflammation.
Saw Palmetto
A Strong clinical evidenceSerenoa repens
Saw palmetto fruit extract may help urinary symptoms of benign prostate enlargement; raised INR has been reported with warfarin.
Ginkgo Leaf
A Strong clinical evidenceGinkgo biloba
Standardised ginkgo extract may help age-related cognitive decline and mild dementia; bleeding risk and interactions call for caution.
Marshmallow Root
T Traditional useAlthaea officinalis
Marshmallow root is soaked in cold water and used to soothe throat irritation with dry cough and mild stomach discomfort.
Psyllium Husk
A Strong clinical evidencePlantago ovata
Taken with plenty of water, psyllium husk helps soften stools in constipation; keep a gap between it and other medicines.
St. John's Wort
A Strong clinical evidenceHypericum perforatum
St John's wort extract may help with mild to moderate depressive episodes, but it seriously interacts with many medicines, including the pill.
Garlic
T Traditional useAllium sativum
Garlic powder is traditionally used to help prevent hardening of the arteries, and garlic extract to relieve symptoms of the common cold.
Willow Bark
A Strong clinical evidenceSalix spp.
Willow bark extract may help with short-term low back pain. It contains salicylates, so it is unsuitable under 18 and with blood thinners.
Green Tea
T Traditional useCamellia sinensis
Green tea is traditionally used to relieve fatigue and a feeling of weakness; it contains caffeine and is unsuitable with high blood pressure or ulcers.