Investigations › Supplements
Investigation No. 11 · Supplements
“Natural means safe, and herbal means harmless”
St John’s wort induces the enzyme that clears roughly half of all prescription drugs. Two heart transplant patients rejected their grafts because of it.
At a glance
- Verdict
- False, and risky
- Certainty in the evidence
- High, starting at Low and moved by 2 domains
- Evidence base
- Pharmacokinetic studies and case series, reviewed 2020
- Last reviewed
- 11 Sept 2026
- Next review due
- Mar 2027
How people actually say it
- “It’s natural, so it can’t hurt.”usually about something bought without a prescription
- “I didn’t think I needed to mention it, it’s just a herb.”said to a pharmacist, after the interaction
Why this verdict and not the next one
Risky rather than mostly false, because the harm is named, documented and severe: graft rejection, contraceptive failure and anticoagulant failure are all consequences somebody has recorded.
Where the claim came from
From a word doing work it was never entitled to. Natural describes where something came from and says nothing about what it does at the dose in the bottle. Digitalis, atropine and warfarin are all natural.
What the studies did
Nicolussi and colleagues reviewed twenty years of St John’s wort interaction data. The mechanism is not mysterious: the preparation activates the pregnane-X-receptor, which induces CYP3A4 and P-glycoprotein, which then clear other drugs faster than intended. The induction tracks the hyperforin content of the specific preparation, which is why two products labelled the same can behave differently.
Why these papers and not others
A narrative review was used rather than a systematic one, because none exists for this question. That is stated rather than hidden, and it is why the certainty starts low.
The full search and appraisal protocol, including what gets excluded and why, is set out in the method.
The evidence, study by study
| Study | Design | n | What it found | What it cannot say |
|---|---|---|---|---|
| Nicolussi 2020 | Narrative review of pharmacokinetic interaction studies | — | St John’s wort activates the pregnane-X-receptor, inducing CYP3A4 and P-glycoprotein. Altered pharmacokinetics of cyclosporine, digoxin, tacrolimus, indinavir, warfarin, alprazolam, simvastatin and oral contraceptives. | How often this happens in practice, since the review collects reported cases rather than counting them in a population |
| Case reports, 2000 | Two case reports | 2 | Acute rejection in two heart transplant patients after a pharmacokinetic interaction with cyclosporine | Frequency, or which patients are most at risk |
How the certainty grade was reached
Certainty starts at a level set by the study design, then moves down for every domain that fails a written threshold and up for every domain that clears one. The arithmetic is shown rather than asserted.
What the evidence cannot say
Tell you the population frequency. These are pharmacokinetic studies and reported cases, not a cohort, so the review establishes that the interaction is real and severe without establishing how many people it reaches.
One person, worked through
Somebody on an oral contraceptive starts St John’s wort for low mood, bought without a prescription and not mentioned at any appointment because it is a herb. The contraceptive is cleared faster. Nothing about that sequence involves anybody behaving unreasonably, and it is the most common version of this harm.
Whether this applies to you
A verdict is a reading of a population. Whether it describes you depends on the things below, and any one of them can change what the right move is for you.
- Anything on a transplant, anticoagulant or antiretroviral regimen is where the consequences are most severe
- Oral contraceptives are the most commonly affected medication in ordinary use
- The specific preparation matters because hyperforin content varies between products
- Tell the pharmacist which is the whole practical answer here, and free
What would change this entry
Population-level pharmacovigilance data, which would turn a documented mechanism into a rate.
Read this before you act on it
This is an appraisal of published evidence, not medical or nutrition advice, and it is not a substitute for care from a clinician who knows your history, your medications and your labs. Nothing here is a diagnosis, a prescription or a treatment plan, and reading it does not create a clinician and patient relationship.
Do not start, stop or change a medication, a supplement or a diet on the strength of this page. If you are pregnant, breastfeeding, managing kidney or liver disease, diabetes, an eating disorder or any chronic condition, the general reading above may not describe you at all. Talk to your physician or a registered dietitian first.
The verdict above was true to the evidence on 11 Sept 2026. Evidence moves. Check the review date.
References
- Nicolussi S, Drewe J, Butterweck V, Meyer zu Schwabedissen HE. Clinical relevance of St John’s wort drug interactions revisited. British Journal of Pharmacology. 2020;177(6):1212–1226. doi:10.1111/bph.14936. PMID 31742659.
