Interactions of St John’s Wort (Hypericum perforatum) preparations

St John’s Wort (SJW) preparations are unlicensed herbal remedies. Their levels of active ingredients can vary from one preparation to another. They are widely used in the UK, being available from pharmacies, health food shops and herbal practitioners.

Why do SJW preparations interact with other medicines?

New evidence suggests that SJW preparations may interact with medicines, either by affecting drug metabolism or levels of neurotransmitters. Drug metabolism may be affected by SJW preparations inducing certain cytochrome P450 isoenzymes in the liver (CYP 3A4, 1A2 and 2C9), as well as, P-glycoprotein. Pharmacodynamic (additive or potentiating) interactions may occur through the effects of SJW preparations on neurotransmitters in the brain (SJW may increase serotonin levels through weak monoamine oxidase inhibiting (MAOI) activity and serotonin re-uptake inhibition).

What is the clinical significance of these interactions?

Induction of drug metabolism increases the breakdown of drugs so reducing their blood levels and therapeutic effects. Because the levels of active ingrediants can vary between preparations of SJW and patients may switch between preparations, the degree of induction is likely to change over time. When patients stop taking SJW preparations, blood levels of interacting medicines may rise, leading to toxicity.

Pharmacodynamic (additive or potentiating) interactions may occur with psychoactive medicines including Selective Serotonin Reuptake Inhibitors (SSRIs). St John’s Wort preparations may also have pharmacodynamic interactions with triptans used to treat migraine. These interactions may result in serious adverse reactions.

Which medicines interact with SJW?

Table 1 (see above – Editor, HTB) lists medicines where in-vitro studies, pharmacokinetic studies or spontaneously reported suspected adverse reactions demonstrate clinically important interactions. In addition, other drugs are included where evidence is lacking but clinically important interactions are likely. Please note that the action of many other drugs depends on their rate of metabolism and thus other drugs may also interact with SJW preparations. In general, the following medicines are not likely to interact with SJW preparations:

  • Topical medicines with limited systemic absorption (inhalers, creams, ointments, eye and ear drops, enemas etc).
  • Non-psychotropic medicines which are renally excreted.

For further information

Reporting suspected adverse reactions including interactions

If you suspect your patient has had an adverse reaction associated with any licensed or unlicensed herbal remedy, including a SJW preparation, then please report this to the MCA/CSM using the Yellow Card Scheme in the normal way. Yellow Cards can be found in the back of the British National Formulary (BNF).

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