A herb that does nothing is a waste of money. A herb that alters your medication is a medical problem. That is why interactions deserve more of your attention than efficacy does, and why this is the part of the supplement conversation worth taking seriously even if you are sceptical of the rest of it.
Why is St John's wort the standard example?
Because the list of medicines it affects is long and consequential. NCCIH documents interactions with amitriptyline, bupropion, birth control pills, cyclosporine, phenytoin, carbamazepine, digoxin, ivabradine, indinavir, nevirapine, irinotecan, imatinib, docetaxel, warfarin and simvastatin.
Read across that list and you are looking at antidepressants, contraception, transplant medication, anti-seizure drugs, heart medication, anticoagulants, HIV medication and chemotherapy agents. Reduced effectiveness in any of those is a serious outcome, and in several of them it is dangerous.
NCCIH also flags a risk that runs the other way: "Taking St. John's wort with certain antidepressants or other drugs that affect serotonin may lead to increased serotonin-related side effects, which can be serious." So it can both weaken some drugs and amplify the effects of others.
Is it only St John's wort?
No, and this is the part people find genuinely surprising. NCCIH's guidance on how herbs interact with medicines notes that "Concentrated green tea supplements can reduce blood levels of some drugs, such as the cholesterol-lowering drug atorvastatin and the beta-blocker nadolol."
Green tea is about as harmless as a supplement sounds. That is precisely the point: the concentration in a supplement is not the concentration in a cup, and a familiar name is no guarantee of an inert product.
| Herb | What NCCIH documents |
|---|---|
| St John's wort | Interactions across antidepressants, contraception, cyclosporine, warfarin, simvastatin, digoxin, HIV and chemotherapy drugs, plus serotonin-related risk |
| Concentrated green tea | Can reduce blood levels of atorvastatin and nadolol |
Why does this happen at all?
Broadly, because your body processes herbs through the same machinery it uses for drugs. When a plant compound speeds that machinery up, the drug clears faster and its blood level falls, which is how a medicine quietly stops working properly. When it slows it down, the drug accumulates. Neither is announced by any symptom you would notice at the time, which is what makes it worth checking in advance rather than waiting to find out.
What should you actually do?
- Tell every provider what you take. NCCIH is direct about this: "When you visit your health care providers, it's important to tell them about all the medicines and supplements you take."
- Keep the list on your phone. Names and strengths, updated when you change something. It takes two minutes and it is the single highest-value habit here.
- Ask a pharmacist before starting anything new. This is free, fast, and exactly what they are trained for.
- Check before surgery. Some supplements affect bleeding and anaesthesia, and your surgical team needs to know well in advance.
- Do not stop prescribed medication to accommodate a supplement. If something conflicts, the supplement is the thing that goes.
The habit that closes most of the risk
People leave supplements off the list when a clinician asks what medication they are taking, because a supplement does not feel like a drug. From an interaction standpoint it can behave like one. Simply including them in the answer resolves most of this category of risk without you needing to memorise anything.
Who should be most careful
- Anyone on any prescription medication, given the breadth of what is documented.
- Anyone taking a drug where the dose is finely balanced, such as warfarin or an anti-seizure medicine.
- Anyone on contraception, given the documented interaction with birth control pills.
- Anyone undergoing cancer treatment or taking immunosuppressants.
- Anyone pregnant, breastfeeding, or approaching surgery.
For the wider picture, including how loosely this whole category is regulated, see our guide to how supplements are actually regulated and our complete guide to herbal remedies.
