St John's Wort Dosage and Its Serious Drug Interactions
St John's wort is the one supplement where the interaction section matters more than the dosing section. It can reduce the effectiveness of a long list of prescription medicines, including contraceptives and transplant drugs.
Depression trials of St John's wort most often used 300 mg three times daily — 900 mg per day — of an extract standardised to about 0.3% hypericin, with some trials using 900 to 1,800 mg per day over four to twelve weeks. The dose is not the important part of this page. St John's wort strongly induces CYP3A4 and the P-glycoprotein transporter, which lowers blood levels of many medicines including hormonal contraceptives, warfarin, ciclosporin, some HIV and cancer drugs, and can cause serotonin syndrome when combined with antidepressants.
St John's wort amounts used in depression trials
| Use case | Amount reported in sources | What to know |
|---|---|---|
| Standard trial regimen | 300 mg three times daily (900 mg/day) | Extract standardised to approximately 0.3% hypericin. Source |
| Higher-dose arms | 900–1,800 mg per day | Used in some trials of moderate depression; typically 4–12 weeks. Source |
| Standardisation markers | Hypericin and hyperforin | Hyperforin drives the enzyme induction responsible for most drug interactions; content varies by product. Source |
| Time to effect in trials | Typically assessed at 4–12 weeks | Not an acute treatment; trials measured symptom scales over weeks. Source |
Figures are amounts reported in the cited sources, not a personal recommendation. Your own amount depends on your health, medicines and blood work.
Start with the interactions, not the dose
St John's wort induces cytochrome P450 3A4 and the P-glycoprotein transporter. In plain terms, it speeds up the body's clearance of many drugs, so those drugs stop working as well.
Contraceptive failure and breakthrough bleeding have been documented with hormonal contraception. Transplant rejection has been reported in patients on ciclosporin. Reduced effectiveness has been reported for warfarin, digoxin, some HIV antiretrovirals, some cancer treatments, statins and certain anticonvulsants.
In the opposite direction, combining it with SSRIs, SNRIs, triptans or other serotonergic drugs can produce serotonin syndrome, which is a medical emergency.
This is why any decision to take St John's wort belongs with a prescriber or pharmacist who can see the whole medicine list — not with a supplement label.
- Induces CYP3A4 and P-glycoprotein
- Documented contraceptive failure
- Documented transplant rejection with ciclosporin
- Serotonin syndrome risk with antidepressants and triptans
What the depression evidence shows
St John's wort has been studied extensively for depression. A Cochrane review of trials in major depression concluded that hypericum extracts were superior to placebo and comparably effective to standard antidepressants, with fewer reported side effects in the included trials.
That finding comes with caveats reviewers stated clearly: results were more favourable in German-speaking countries where the preparations originated, trials varied in quality, and studies were mostly short and in mild to moderate depression.
Authoritative bodies do not recommend it for severe depression, and it is not a substitute for assessment when someone is unwell. Depression that warrants treatment warrants a clinician.
Hyperforin, product variability and why brands differ
Hypericin is the traditional standardisation marker, but hyperforin is the constituent most responsible for enzyme induction. Products differ substantially in hyperforin content, and low-hyperforin preparations have been developed specifically to reduce interactions.
That variability cuts both ways: a low-hyperforin product may interact less but may also differ from the extracts used in the effectiveness trials.
Because labels rarely state hyperforin content, assume interaction potential is present unless a pharmacist confirms otherwise.
Side effects beyond interactions
The commonly reported effects are photosensitivity — increased sunburn risk, especially in fair skin — along with dry mouth, dizziness, gastrointestinal symptoms, fatigue and headache.
Anxiety, restlessness and sleep disturbance are also reported. In people with bipolar disorder, antidepressant-type treatments including St John's wort carry a risk of triggering mania.
Stopping abruptly after prolonged use has been associated with discontinuation symptoms, so tapering with clinical advice is sensible.
If you are considering it
Bring your full medicine list — prescriptions, contraception, supplements — to a pharmacist before starting. This is the single most useful step, and it takes minutes.
Do not combine it with an antidepressant, and do not stop a prescribed antidepressant to switch to it without supervision.
If you do take it, record it in your medicine list so every clinician who sees you knows. It is the supplement most likely to change how a prescription behaves.
What the published studies found
Each entry below links straight to the paper or the health authority page so you can read the original rather than take our word for it.
- National Center for Complementary and Integrative Health. St. John's Wort.
Authoritative evidence and safety summary
Documents that St John's wort can interact with many medicines, reducing effectiveness of hormonal contraceptives, ciclosporin, warfarin, some HIV and cancer drugs, and can cause serotonin syndrome with antidepressants.
- Linde K et al. St John's wort for major depression. Cochrane Database of Systematic Reviews.
Systematic review and meta-analysis of randomised trials
Hypericum extracts were superior to placebo and similarly effective to standard antidepressants in the included trials, with fewer reported adverse effects; reviewers noted trial quality and country-of-origin differences.
- Randomised trials of 900 mg/day standardised hypericum extract.
Randomised, placebo- and comparator-controlled trials, 4–12 weeks
The 300 mg three-times-daily regimen of extract standardised to about 0.3% hypericin is the most frequently used protocol in the depression literature.
Side effects reported in the literature
Commonly reported
- Photosensitivity and increased sunburn risk
- Dry mouth
- Dizziness
- Gastrointestinal symptoms
- Fatigue and headache
- Restlessness or sleep disturbance
Serious, seek medical advice
- Serotonin syndrome when combined with serotonergic medicines
- Loss of effectiveness of many prescription drugs through enzyme induction
- Possible triggering of mania in bipolar disorder
Summarized from NCCIH — St. John's Wort. Report anything unexpected to your own doctor or pharmacist.
Interactions to check with a pharmacist
| Medicine or condition | What sources report |
|---|---|
| Hormonal contraceptives | Reduced effectiveness and breakthrough bleeding have been documented. Source |
| Ciclosporin and other transplant medicines | Lowered drug levels; transplant rejection has been reported. Source |
| SSRIs, SNRIs, triptans and other serotonergic drugs | Risk of serotonin syndrome, a medical emergency. Source |
| Warfarin, digoxin, HIV antiretrovirals, some cancer drugs and anticonvulsants | Enzyme and transporter induction can lower blood levels and reduce effectiveness. Source |
This is not a complete interaction list. Check your own medicines with a pharmacist before combining anything.
This page is reference material, not medical advice. Supplements interact with prescription medicines and with each other. Talk to a doctor or pharmacist before starting anything, especially if you are pregnant, breastfeeding, managing a health condition or taking prescription medicine.
Frequently asked questions
How much St John's wort was used in depression trials?
Most trials used 300 mg three times daily — 900 mg per day — of an extract standardised to about 0.3% hypericin, assessed over four to twelve weeks.
Does St John's wort stop birth control from working?
It can. Reduced contraceptive effectiveness and breakthrough bleeding have been documented, because St John's wort speeds up the breakdown of hormonal contraceptives.
Can I take St John's wort with an antidepressant?
No, not without prescriber supervision. Combining it with SSRIs, SNRIs, triptans or other serotonergic medicines can cause serotonin syndrome, which is a medical emergency.
Is St John's wort effective for depression?
A Cochrane review found hypericum extracts superior to placebo and comparable to standard antidepressants in mild to moderate depression, with caveats about trial quality. It is not recommended for severe depression and should not replace clinical assessment.
Keep reading
Sources
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