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St John’s Wort (Hypericum perforatum) is a herbal medicinal product commonly used for the management of mild to moderate depressive symptoms. Its widespread availability and use alongside prescribed medicines have raised clinically relevant concerns regarding drug–herb interactions.
Evidence suggests that St John’s wort interacts with a broad range of medicines through effects on drug-metabolising enzymes and transporters, with clinically significant consequences in patients receiving long-term pharmacotherapy [1][2][3][4].
Clinical relevance and prevalence of interactions
Clinical interaction studies, case reports, and pharmacovigilance data show that St John’s wort can alter the pharmacokinetics of a wide range of medication, particularly those dependent on cytochrome P450 isoenzymes and P-glycoprotein (P-gp) for metabolism and transport [2][3].
Under-reporting of herbal medicine use by patients has been documented, increasing the likelihood that St John’s wort is co-administered unintentionally with interacting therapies [2]. This is clinically relevant in settings where loss of therapeutic efficacy may result in serious or life-threatening outcomes [1][2][3][4].
Pharmacokinetic mechanisms
Hyperforin, a key constituent of St John’s wort, activates nuclear receptors involved in transcriptional up-regulation of cytochrome P450 enzymes [1][3]. The most notable effect is the induction of CYP3A4 [5].
P-glycoprotein, which mediates intestinal and hepatic drug transport, is also induced by St John’s wort [1][3].
This dual mechanism suggests that St John’s wort may reduce systemic exposure to a wide range of drugs. As a result, long-term use may compromise therapeutic efficacy or necessitate dose adjustments for many medications [3][5].
Pharmacodynamic mechanisms
In addition to pharmacokinetic effects, St John’s wort possesses intrinsic serotonergic activity. Concomitant use with other serotonergic agents may result in additive pharmacodynamic effects and an increased risk of serotonin-mediated adverse reactions [1][3][4].
Risk amplification during long-term pharmacotherapy
The clinical impact of St John’s wort–drug interactions is most pronounced in patients receiving long-term treatment, particularly when therapeutic efficacy depends on maintaining stable plasma drug concentrations [1][2][3][4]. Induction of metabolic enzymes and transporters develops progressively over days to weeks and may persist after discontinuation [5], meaning that interaction effects may not be immediately apparent and can lead to delayed recognition.
Patients at greatest risk of clinically significant harm include those receiving narrow therapeutic index medicines, those dependent on stable plasma concentrations for disease control, and those undergoing chronic or lifelong pharmacotherapy [1][2][3][4].
Risk is further amplified when St John’s wort is initiated or discontinued without clinician awareness. If doses of co‑administered drugs were increased to compensate for reduced exposure during St John’s wort co‑administration, discontinuation of the herb can result in transiently elevated drug concentrations and risk of toxicity [2][3].
Medicines associated with clinically important interactions
Hormonal contraceptives
Clinical pharmacokinetic studies show that St John’s wort reduces plasma concentrations of estrogens and progestogens when co-administered with combined oral contraceptives.
This interaction has been associated with breakthrough bleeding and evidence of ovulatory activity, indicating reduced contraceptive efficacy [2].
- Mechanism: CYP3A4 induction increases steroid metabolism
- Clinical consequence: reduced contraceptive effectiveness, possibly leading to an increased risk of unintended pregnancy
Immunosuppressants
Reductions in ciclosporin and tacrolimus concentrations have been reported in patients taking St John’s wort concurrently, with documented cases of loss of immunosuppressive control [2][3].
- Mechanism: CYP3A4 and P-gp induction
- Clinical consequence: increased risk of transplant rejection due to subtherapeutic immunosuppressant concentrations, necessitating more frequent therapeutic drug monitoring [2][3][4]
Antiretroviral agents
Human interaction studies show that St John’s wort reduces exposure to HIV protease inhibitors (e.g. indinavir) and non‑nucleoside reverse transcriptase inhibitors (NNRTIs; e.g. nevirapine) [3].
- Mechanism: CYP3A4 induction
- Clinical consequence: reduced antiviral efficacy, with risk of virological failure and resistance
Oral Anticoagulants
St John’s wort and oral anticoagulants (e.g. warfarin) have been associated with variable effects on anticoagulation, attributed to increased metabolic clearance [2][3].
- Mechanism: enzyme induction
- Clinical consequence: inconsistent anticoagulant response with potential for reduced anticoagulation and increased thrombotic risk; requires more frequent INR monitoring
Cardiac Inotropic Drugs
Digoxin concentrations decrease during concurrent use of St John’s wort, primarily due to induction of intestinal P-glycoprotein [2][3].
- Mechanism: P-gp induction
- Clinical consequence: loss of rate or symptom control
Antidepressants and other serotonergic agents
Case reports and reviews have documented adverse serotonergic effects when St John’s wort is combined with selective serotonin reuptake inhibitors (SSRIs), serotonin–noradrenaline reuptake inhibitors (SNRIs), or triptans [1][3].
- Mechanism: additive serotonergic activity
- Clinical consequence: increased risk of serotonin-mediated adverse effects
Opioids
Pharmacokinetic studies indicate that St John’s wort reduces plasma concentrations of opioids (e.g. methadone, oxycodone) [1][3].
- Mechanism: enzyme induction
- Clinical consequence: reduced efficacy or opioid withdrawal symptoms
Clinically relevant interactions involving St John’s wort are often identified only after loss of therapeutic efficacy has occurred, highlighting the need for proactive risk mitigation [1].
Evidence-supported strategies include:
- Routine enquiry about herbal medicine use during medication reviews, as St John’s wort use is frequently undisclosed unless specifically requested [2].
- Patient counselling prior to initiation, particularly for those receiving medicines where reduced exposure may result in treatment failure or serious clinical consequences [1].
- Monitoring for reduced therapeutic effect in patients receiving long-term pharmacotherapy, as interactions may present without overt adverse effects [1].
- Recognition of delayed and persistent interaction effects, given that enzyme induction may take several days to develop and can persist after St John’s wort discontinuation [1].
- Avoiding use with high-risk medicines, such as immunosuppressants, antiretrovirals, and other drugs where even modest reductions in plasma concentrations may have serious clinical consequences [1][2].
- Special caution for drugs with a narrow therapeutic index, where small fluctuations in plasma levels can lead to therapeutic failure or toxicity; these patients should be closely monitored if concomitant use is unavoidable [2][3].
These measures support earlier identification of interactions and reduce the risk of preventable loss of disease control in susceptible patients [1].
Integration with MIMS Clinical Decision Solutions
MIMS Clinical Decision Solutions supports identification and management of St John’s wort–related interactions at the point of prescribing.
The Drug Interaction Alert module flags clinically significant interactions and provides evidence-based recommendations on their management.
The Generic Monograph module highlights known interactions with St John’s wort within individual monographs and advises on avoidance or monitoring where appropriate.
Conclusion
St John’s wort is associated with clinically significant interactions across multiple therapeutic classes, primarily through induction of drug-metabolising enzymes and transporters and through additive serotonergic effects.
Recognition of St John’s wort use, structured medication review, and integration of clinical decision support tools are essential to maintaining therapeutic efficacy and prescribing consistency in patients receiving long-term pharmacotherapy.
References
- Steenkamp V, Parkar H, Dasgupta A. Utility of Therapeutic Drug Monitoring in Identifying Clinically Significant Interactions Between St. John’s Wort and Prescription Drugs. Ther Drug Monit. 2023 Feb 1;45(1):35-44. See more
- Mannel M. Drug interactions with St John’s wort: mechanisms and clinical implications. Drug Saf. 2004;27(11):773-97. See more
- Zhou S, Chan E, Pan SQ et al. Pharmacokinetic interactions of drugs with St John’s wort. J Psychopharmacol. 2004 Jun;18(2):262-76. See more
- Borrelli F, Izzo AA. Herb–drug interactions with St John’s wort (Hypericum perforatum): an update on clinical observations. *AAPS J.* 2009 Dec;11(4):710–27. See more
- Markowitz JS, Donovan JL, DeVane CL et al. Effect of St John’s wort on drug metabolism by induction of cytochrome P450 3A4 enzyme. JAMA. 2003 Sep 17;290(11):1500-4. See more