St. John's Wort — Critical Drug Interactions

⚠ CRITICAL SAFETY WARNING

St. John's Wort has the most extensive and clinically significant drug-interaction profile of any commonly used medicinal herb. Documented cases include unintended pregnancies in women taking oral contraceptives, acute organ rejection in transplant patients, HIV antiretroviral treatment failure with viral resistance, warfarin anticoagulation failure with thrombotic events, and life-threatening serotonin syndrome from combination with SSRIs. The FDA, European Medicines Agency, Health Canada, the UK Medicines and Healthcare products Regulatory Agency, and Australia's Therapeutic Goods Administration have all issued formal regulatory warnings. Before starting St. John's Wort, review every prescription, over-the-counter, and herbal supplement you take with a qualified pharmacist or physician. Before starting any new medication while on St. John's Wort, disclose its use to the prescriber. Discontinue at least 5 days before any planned surgery.

The same hyperforin molecule that drives St. John's Wort's antidepressant efficacy also produces its catastrophic drug-interaction profile. Hyperforin is a potent agonist of the pregnane X receptor (PXR), which is the master transcription factor controlling expression of the CYP3A4, CYP1A2, CYP2C9, and CYP2C19 cytochrome P450 enzymes and the P-glycoprotein drug efflux transporter. PXR activation by hyperforin upregulates these drug-metabolizing systems by 50-300% over baseline, dramatically accelerating the clearance of any drug that is a substrate. More than half of all prescription drugs in clinical use are metabolized by CYP3A4 alone — including oral contraceptives, anticoagulants, immunosuppressants, HIV antiretrovirals, cardiac drugs, chemotherapy, statins, anticonvulsants, and many psychiatric medications. The clinical consequences range from contraceptive failure (with documented unintended pregnancies) to organ-rejection cases in transplant patients to antiretroviral treatment failure with viral resistance to anticoagulation failure with thrombotic events. Separately, the herb's pharmacodynamic interaction with serotonergic drugs (SSRIs, SNRIs, triptans, tramadol, MAOIs) can produce potentially fatal serotonin syndrome. This deep-dive walks through the molecular mechanism, the comprehensive interaction tables, the FDA and international regulatory warning history, and the practical clinical-decision framework for any patient considering or already taking St. John's Wort.

Table of Contents

  1. Molecular Mechanism — PXR Activation by Hyperforin
  2. Comprehensive CYP3A4 Interaction Table
  3. Oral Contraceptives — Documented Contraceptive Failures
  4. Warfarin and Anticoagulants
  5. Immunosuppressants — Organ Rejection Cases
  6. HIV Antiretrovirals — Absolute Contraindication
  7. Serotonin Syndrome with SSRIs/SNRIs/Triptans
  8. Cardiac Drugs (Digoxin, Statins, Antiarrhythmics)
  9. Chemotherapy and Targeted Cancer Therapies
  10. Anticonvulsants and Psychiatric Medications
  11. FDA, EMA, and Health Canada Warning History
  12. Surgical and Anesthetic Considerations
  13. Clinical Decision Framework
  14. Key Research Papers
  15. Connections

Molecular Mechanism — PXR Activation by Hyperforin

The pharmacological root of St. John's Wort's drug-interaction profile is the activation of the pregnane X receptor (PXR) by hyperforin, the herb's principal antidepressant constituent. PXR (also known as NR1I2 or the steroid and xenobiotic receptor SXR) is a nuclear hormone receptor that functions as the master transcription factor controlling expression of the body's xenobiotic metabolism and elimination machinery.

When PXR is activated by an agonist (endogenous bile acids, hyperforin, the antibiotic rifampin, the antifungal clotrimazole, and various other xenobiotics), it translocates to the nucleus, heterodimerizes with the retinoid X receptor (RXR), and binds to PXR response elements (DR3 motifs) in the promoter regions of target genes including:

The induction kinetics matter clinically:

The clinical consequence: any drug whose effective plasma concentration depends on CYP3A4, CYP1A2, CYP2C9, CYP2C19, P-gp, or UGT metabolism will have reduced plasma levels and reduced clinical effect when hypericum is co-administered. The magnitude of the interaction varies by drug (some have plasma level reductions of 25%, others of 75% or more), but the direction is consistent: hypericum reduces drug exposure.

Separately, hyperforin's monoamine reuptake inhibition (the mechanism behind its antidepressant effect) creates a pharmacodynamic interaction with other serotonergic, noradrenergic, and dopaminergic drugs — the effects of those drugs are additive or synergistic with hypericum's. The most clinically dangerous example is serotonin syndrome with SSRI/SNRI/triptan/MAOI combinations.

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Comprehensive CYP3A4 Interaction Table

The following table summarizes the most clinically significant CYP3A4-mediated and pharmacodynamic interactions with St. John's Wort. Categories of severity:

Hormonal Contraceptives

Anticoagulants and Antiplatelets

Immunosuppressants (Organ Transplant)

HIV Antiretrovirals

Cardiovascular Medications

Psychiatric Medications (Pharmacodynamic Serotonin Syndrome Risk)

Chemotherapy and Targeted Cancer Therapies

Anticonvulsants

Other Important Interactions

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Oral Contraceptives — Documented Contraceptive Failures

The hypericum-contraceptive interaction is among the most consequential because the failure mode is a permanent life-altering event (unintended pregnancy) that often occurs without warning — the woman has no way to know contraceptive efficacy is reduced until breakthrough bleeding occurs or, worse, until pregnancy is confirmed.

The mechanism: combined oral contraceptive pills contain ethinyl estradiol (or in newer formulations, estetrol or estradiol valerate) plus a progestin (levonorgestrel, norethindrone, drospirenone, etc.). Both estrogen and progestin components undergo CYP3A4 metabolism in the gut wall and liver, and both undergo intestinal P-glycoprotein efflux. Hyperforin's induction of CYP3A4 and P-gp accelerates the metabolism and reduces the absorption of both components.

The clinical consequences:

The literature includes multiple case reports and case series of pregnancies in women who became pregnant despite consistent oral contraceptive use after starting hypericum. The German pharmacovigilance database accumulated several documented cases in the late 1990s and early 2000s, which contributed to European Medicines Agency safety communications. The UK Medicines and Healthcare products Regulatory Agency issued a formal warning in 2000. The U.S. FDA followed in 2000 with its first formal hypericum-interaction advisory.

Practical guidance for women on hormonal contraception who are considering hypericum:

  1. If pregnancy prevention is essential, do not use hypericum with combined oral contraceptive pills, progestin-only pills, the patch, the ring, or the etonogestrel implant. Use an alternative depression treatment
  2. If hypericum is essential and the woman is on hormonal contraception, switch to a non-hormonal method (copper IUD) or a method with lower interaction risk (levonorgestrel IUD) before starting hypericum, and continue the alternative method for at least 2 weeks after stopping hypericum
  3. For women currently on the combination who cannot or will not change either treatment, use barrier contraception (condoms) consistently as backup. Counseling about the increased pregnancy risk and the option of post-coital emergency contraception (with awareness that even emergency contraception efficacy may be reduced)
  4. The 2-week washout period: CYP3A4 induction takes approximately 2-3 weeks to dissipate after hypericum discontinuation. Contraceptive efficacy may not return to normal for several weeks after stopping hypericum, even if the contraceptive method has not been changed

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Warfarin and Anticoagulants

Warfarin is a vitamin-K-antagonist anticoagulant used for stroke prevention in atrial fibrillation, treatment of venous thromboembolism (deep vein thrombosis and pulmonary embolism), and prevention of clot formation in mechanical heart valves. Therapeutic warfarin dosing requires a delicate balance between adequate anticoagulation (typically targeting an INR of 2.0-3.0, or 2.5-3.5 for mechanical valves) and excessive anticoagulation (with bleeding risk).

Hypericum interaction with warfarin:

Management:

Direct oral anticoagulants (DOACs):

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Immunosuppressants — Organ Rejection Cases

Solid organ transplant recipients (kidney, liver, heart, lung, pancreas, small bowel) require lifelong immunosuppression to prevent allograft rejection. Standard regimens typically include a calcineurin inhibitor (cyclosporine or tacrolimus), often combined with mycophenolate and corticosteroids, with maintenance dosing carefully titrated based on therapeutic drug monitoring to keep trough levels within narrow target ranges.

The hypericum-immunosuppressant interaction is among the most catastrophic:

The clinical recommendation is unambiguous: hypericum is ABSOLUTELY CONTRAINDICATED in solid organ transplant recipients. The interaction has caused documented allograft loss and is not manageable with even intensive therapeutic drug monitoring. Patients with depression after transplantation should use conventional pharmaceutical antidepressants with known interaction profiles (avoiding strong CYP3A4 substrates or inhibitors) or non-pharmaceutical interventions (CBT, exercise, light therapy if seasonal).

Stem cell transplant and hematopoietic cell transplant recipients on immunosuppression for graft-versus-host disease prophylaxis or treatment face the same restrictions.

Patients on biologic immunomodulators (TNF inhibitors, JAK inhibitors, IL-17 inhibitors, IL-23 inhibitors) for autoimmune disease generally have lower interaction risk because most biologics are not CYP-metabolized, though specific interactions should be reviewed for each agent.

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HIV Antiretrovirals — Absolute Contraindication

The hypericum-HIV antiretroviral interaction was the trigger for the first formal FDA warning about hypericum in February 2000. The seminal study was Piscitelli et al. (Lancet, February 2000), which examined the effect of 14 days of hypericum administration on indinavir pharmacokinetics in healthy volunteers. The results were dramatic:

The clinical consequence: patients on indinavir who add hypericum will experience loss of viral suppression, viral load rebound, and emergence of HIV protease resistance mutations that may compromise future treatment options. The same magnitude of interaction has been documented with other protease inhibitors and with non-nucleoside reverse transcriptase inhibitors (NNRTIs) including efavirenz and nevirapine.

Integrase strand transfer inhibitors (INSTIs) such as dolutegravir, elvitegravir/cobicistat, raltegravir, and bictegravir are increasingly the first-line antiretroviral class, but they are also significantly affected by hypericum (dolutegravir and bictegravir are UGT substrates that may be affected; cobicistat-boosted regimens lose pharmacokinetic boosting).

The FDA, Health Canada, and European Medicines Agency all maintain warnings that hypericum is contraindicated for use with HIV antiretroviral medications. This applies to all currently marketed antiretroviral classes:

HIV-positive patients with depression should be referred to HIV specialty care for selection of an antidepressant compatible with their antiretroviral regimen. Common options include bupropion, mirtazapine, or specific SSRIs known to have lower CYP3A4 interaction (e.g., citalopram or escitalopram, with caution for QT prolongation when combined with certain antiretrovirals).

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Serotonin Syndrome with SSRIs/SNRIs/Triptans

Beyond the pharmacokinetic CYP-induction interactions discussed above, hypericum produces a clinically critical pharmacodynamic interaction with other serotonergic drugs. The mechanism: hyperforin's inhibition of serotonin reuptake (via the TRPC6/sodium-gradient mechanism) adds to the serotonergic effect of pharmaceutical SSRIs and SNRIs. When combined with other drugs that increase serotonergic signaling (triptans, tramadol, dextromethorphan, MAO inhibitors), the additive effect can produce serotonin syndrome — a potentially fatal hyperthermic crisis.

The clinical features of serotonin syndrome are classified into the Hunter Criteria (the modern diagnostic standard) and include:

Onset is typically within hours to days of adding a second serotonergic agent. The severity ranges from mild (agitation, mild tachycardia, modest tremor) to life-threatening (severe hyperthermia, rigidity, seizures).

Drugs that produce serotonin syndrome risk in combination with hypericum:

Management of suspected serotonin syndrome:

  1. Discontinue all serotonergic agents immediately
  2. Supportive care: IV fluids, cooling for hyperthermia, benzodiazepines (lorazepam) for agitation and tremor
  3. Severe cases: ICU admission, cyproheptadine (a serotonin 5-HT2A antagonist that may be used as antidote), aggressive cooling, intubation if needed
  4. Most cases resolve within 24-72 hours of stopping serotonergic agents

Prevention is far better than treatment: do not combine hypericum with any other serotonergic medication. Transitions between hypericum and pharmaceutical SSRIs should include a washout period of at least 2 weeks in either direction (longer for fluoxetine which has a very long half-life). For more on the cluster of related conditions, see our Depression page.

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Cardiac Drugs (Digoxin, Statins, Antiarrhythmics)

Digoxin: digoxin is a positive inotrope used in heart failure and rate control in atrial fibrillation. It has a narrow therapeutic index (target trough 0.5-2.0 ng/mL) and is primarily a P-glycoprotein substrate (not CYP-metabolized). Hypericum's P-gp induction can reduce digoxin levels by 25-30%, with risk of loss of rate control in atrial fibrillation or worsening heart failure. Co-administration generally avoided; if necessary, digoxin levels monitored closely.

Statins: cholesterol-lowering HMG-CoA reductase inhibitors. Susceptibility to hypericum interaction varies by specific statin:

Calcium channel blockers: most CCBs (amlodipine, nifedipine, felodipine, diltiazem, verapamil) are CYP3A4 substrates with reduced antihypertensive effect possible with hypericum co-administration. Monitor blood pressure if combination is used.

Antiarrhythmics: amiodarone, quinidine, and dronedarone are CYP3A4 substrates with reduced antiarrhythmic effect possible. Amiodarone is particularly problematic because its long half-life means CYP induction effects persist for weeks after hypericum discontinuation.

Ivabradine: a sinus-node funny-current (If) inhibitor used in heart failure; significant CYP3A4 substrate with reduced efficacy with hypericum co-administration. Co-administration discouraged.

Beta-blockers: most beta-blockers have lower CYP3A4 dependence and lower interaction risk with hypericum. Carvedilol has some CYP2D6 metabolism but is less affected than CYP3A4 substrates.

Aspirin and NSAIDs: low interaction risk pharmacokinetically, but combination with hypericum theoretically increases bleeding risk via additive antiplatelet and platelet-function effects (hypericum has weak antiplatelet effect via several mechanisms). Monitor for bleeding if combination is used.

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Chemotherapy and Targeted Cancer Therapies

Patients on active cancer treatment face particular risk from hypericum interactions because many chemotherapy agents and most targeted molecular therapies are CYP3A4 substrates with narrow therapeutic indices. Reduced plasma levels of chemotherapy mean reduced anticancer effect, potentially compromising treatment success.

Specific concerns:

The general principle for cancer patients: do not start hypericum during active anticancer treatment. For depression management in cancer patients, conventional antidepressants (citalopram, escitalopram, mirtazapine, bupropion) with known interaction profiles are preferred. Cancer patients in survivorship who are stable on adjuvant therapy or surveillance should consult their oncology team before starting hypericum.

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Anticonvulsants and Psychiatric Medications

Anticonvulsants used both for seizure disorders and increasingly for mood stabilization (bipolar disorder), neuropathic pain, and migraine prophylaxis have complex interactions with hypericum:

The combination of hypericum with anticonvulsants used for mood stabilization in bipolar disorder is doubly problematic: (1) the pharmacokinetic interaction reduces mood stabilizer effectiveness, and (2) hypericum itself, like other antidepressants, can precipitate manic episodes in bipolar patients. Hypericum is generally contraindicated in bipolar disorder.

Antipsychotic medications:

The recommendation for any patient on antipsychotic therapy for schizophrenia, schizoaffective disorder, or bipolar disorder is to avoid hypericum. The risk of psychiatric decompensation from sub-therapeutic antipsychotic levels is high and not easily monitored clinically.

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FDA, EMA, and Health Canada Warning History

The international regulatory response to hypericum drug interactions provides important context:

The regulatory warnings have been consistent in their message: any patient taking prescription medications should consult a healthcare provider before starting St. John's Wort, and any healthcare provider seeing a patient on St. John's Wort should review all concurrent medications for potential interactions before continuing or prescribing additional therapy.

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Surgical and Anesthetic Considerations

St. John's Wort interactions with anesthetic and surgical medications create specific perioperative concerns:

The standard recommendation from the American Society of Anesthesiologists and most surgical societies is to discontinue St. John's Wort at least 5 days, ideally 2 weeks, before any planned surgery. The 5-day minimum allows substantial reversal of hyperforin-mediated TRPC6/monoamine effects and partial reversal of CYP3A4 induction. The 2-week ideal allows more complete reversal of CYP3A4 induction.

For emergency surgery in patients on hypericum, the surgical and anesthetic team should be informed so that drug doses and choices can be adjusted accordingly.

Postoperative resumption of hypericum: can typically be resumed once the patient has stabilized postoperatively (1-2 weeks), is off most acute pain medications, and is not on any of the contraindicated postoperative drugs (e.g., warfarin for DVT prophylaxis, certain antibiotics, certain analgesics).

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Clinical Decision Framework

For any patient considering St. John's Wort, the following decision framework applies:

  1. Comprehensive medication review: list every prescription medication, every over-the-counter medication, every herbal supplement, every vitamin, and every recreational substance the patient uses. Bring this list to a pharmacist or physician for drug-interaction review. Many drug-interaction databases (Lexi-Interact, Micromedex, Stockley's Drug Interactions) include hypericum and will flag potential interactions
  2. Identify any ABSOLUTELY CONTRAINDICATED combinations: HIV antiretrovirals, immunosuppressants in transplant patients, MAO inhibitors, active chemotherapy. If any of these are present, hypericum cannot be used — consider alternative depression treatment
  3. Identify STRONGLY DISCOURAGED combinations: SSRIs, SNRIs, tricyclics, triptans, tramadol, warfarin (and other anticoagulants), hormonal contraception in women who depend on it for pregnancy prevention. If any of these are present, consider whether the patient can transition off the problematic medication or whether alternative depression treatment is preferable
  4. Identify SIGNIFICANT INTERACTIONS: cardiac medications, statins, anticonvulsants, antipsychotics, calcium channel blockers, theophylline, methadone, sildenafil, etc. These are often manageable with awareness and monitoring, but require informed decision-making
  5. Counsel the patient: explain that the herb has more drug interactions than any other commonly used supplement; that the interactions are real and clinically consequential, not theoretical; that the patient must disclose use to every healthcare provider, dentist, and pharmacist they encounter; that any new medication added during hypericum use requires prescriber awareness; that surgery requires 5-day pre-operative discontinuation; that the 2-week onset window applies (cannot expect immediate effect)
  6. Plan monitoring: for patients on warfarin, INR every 1-2 weeks initially; for transplant patients (where hypericum should not be used at all), trough drug levels; for HIV patients (where hypericum is contraindicated), viral load monitoring
  7. Plan discontinuation strategy: how long is the patient planning to take hypericum? What new medications might be needed during that time? What if the patient gets sick and needs antibiotic therapy or other acute drug treatment? Build the discontinuation plan into the initiation plan

For the prudent patient, the decision tree often comes down to: if any of the absolutely-contraindicated or strongly-discouraged medications are present, do not use hypericum — choose another treatment. The clinical efficacy of hypericum for mild-moderate depression is real, but it is not so dramatically superior to other options (conventional pharmaceuticals, other herbal antidepressants like Saffron, lifestyle interventions, CBT, exercise) that it justifies high-risk drug interactions.

For patients on no other medications, hypericum at standard 900 mg/day dosing of a quality standardized extract is well-tolerated and effective for mild-to-moderate depression, mood symptoms, mild seasonal affective disorder, and menopausal mood symptoms. The interaction profile becomes relevant only when other medications enter the picture.

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Key Research Papers

  1. Piscitelli SC, Burstein AH, Chaitt D, Alfaro RM, Falloon J (2000). Indinavir concentrations and St John's wort. The Lancet, 355(9203):547-8. The pivotal indinavir interaction paper that triggered the FDA warning. — PubMed: Piscitelli 2000 indinavir
  2. Moore LB, Goodwin B, Jones SA, Wisely GB, Serabjit-Singh CJ, Willson TM, Collins JL, Kliewer SA (2000). St. John's wort induces hepatic drug metabolism through activation of the pregnane X receptor. PNAS, 97(13):7500-2. The PXR/CYP3A4 mechanism paper. — PubMed: Moore PXR 2000
  3. Markowitz JS, Donovan JL, DeVane CL, Taylor RM, Ruan Y, Wang JS, Chavin KD (2003). Effect of St John's wort on drug metabolism by induction of cytochrome P450 3A4 enzyme. JAMA, 290(11):1500-4. JAMA mechanism paper. — PubMed: Markowitz JAMA 2003
  4. Henderson L, Yue QY, Bergquist C, Gerden B, Arlett P (2002). St John's wort (Hypericum perforatum): drug interactions and clinical outcomes. British Journal of Clinical Pharmacology, 54(4):349-56. Comprehensive interaction review. — PubMed: Henderson BJCP review
  5. Mannel M (2004). Drug interactions with St John's wort: mechanisms and clinical implications. Drug Safety, 27(11):773-97. Drug Safety review. — PubMed: Mannel Drug Safety 2004
  6. Schwarz UI, Buschel B, Kirch W (2003). Unwanted pregnancy on self-medication with St John's wort despite hormonal contraception. British Journal of Clinical Pharmacology, 55(1):112-3. Documented contraceptive failure case. — PubMed: Schwarz contraceptive failure
  7. Ruschitzka F, Meier PJ, Turina M, Lüscher TF, Noll G (2000). Acute heart transplant rejection due to Saint John's wort. The Lancet, 355(9203):548-9. Documented heart transplant rejection. — PubMed: Ruschitzka heart rejection
  8. Yue QY, Bergquist C, Gerden B (2000). Safety of St John's wort (Hypericum perforatum). The Lancet, 355(9203):576-7. Swedish pharmacovigilance experience with warfarin and other interactions. — PubMed: Yue 2000 safety
  9. Lantz MS, Buchalter E, Giambanco V (1999). St. John's wort and antidepressant drug interactions in the elderly. Journal of Geriatric Psychiatry and Neurology, 12(1):7-10. Serotonin syndrome case series. — PubMed: Lantz serotonin syndrome
  10. Hall SD, Wang Z, Huang SM, Hamman MA, Vasavada N, Adigun AQ, Hilligoss JK, Miller M, Gorski JC (2003). The interaction between St John's wort and an oral contraceptive. Clinical Pharmacology and Therapeutics, 74(6):525-35. Detailed pharmacokinetic study of OC interaction. — PubMed: Hall OC interaction
  11. Johne A, Brockmöller J, Bauer S, Maurer A, Langheinrich M, Roots I (1999). Pharmacokinetic interaction of digoxin with an herbal extract from St John's wort (Hypericum perforatum). Clinical Pharmacology and Therapeutics, 66(4):338-45. Digoxin interaction characterization. — PubMed: Johne digoxin
  12. Mai I, Bauer S, Störmer E, Kuchler A, Mueller-Oerlinghausen B, Roots I (2004). Hyperforin content determines the magnitude of the St John's wort-cyclosporine drug interaction. Clinical Pharmacology and Therapeutics, 76(4):330-40. Hyperforin dose-response in cyclosporine interaction. — PubMed: Mai cyclosporine

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