St John's wort

How it works

Its antidepressant activity is attributed to constituents such as hyperforin and hypericin, which are thought to inhibit reuptake of several monoamines, though the precise mechanism is not fully defined.

Prescribing in practice

• It is a potent inducer of CYP3A4 and P-glycoprotein and can reduce the effectiveness of many medicines, including combined hormonal contraceptives, ciclosporin, anticoagulants and some antiretrovirals and anticonvulsants.
• Combined with serotonergic antidepressants it may precipitate serotonin syndrome.
• Preparations vary widely in active content, so effects and interactions are unpredictable and patients may not disclose its use.

Monitoring

Review all concurrent medicines for loss of efficacy when St John's wort is started or stopped, and watch for serotonergic effects with other antidepressants.

Counselling the patient

• Always tell your doctor or pharmacist if you are taking St John's wort, as it interacts with many prescribed medicines.
• It can make hormonal contraception less reliable, so discuss alternative contraception.
• Do not start or stop it without advice if you take other regular medicines.

Evidence & guidelines

MHRA and NICE highlight the clinically important enzyme-inducing interactions of St John's wort, and NICE does not recommend its prescribing for depression.