St John's wort and SSRI antidepressants: why the herbal remedy is not a harmless extra

St John's wort is a herbal supplement that many people associate with mood. The NHS verapamil page describes it as a herbal remedy for depression, and it is easy to find in pharmacies, supermarkets and online. Because it is sold as a supplement, it can feel gentler than a prescribed tablet. Yet the NHS pages treat it very differently from a vitamin: it appears by name in advice for several prescription medicines.

SSRIs, or selective serotonin reuptake inhibitors, are the most widely used type of antidepressant, according to the NHS. Sertraline, citalopram, escitalopram, fluoxetine and paroxetine are the names it lists. SNRIs such as venlafaxine and duloxetine act on serotonin and noradrenaline. The NHS says these medicines are thought to work by affecting chemical messengers in the brain, including serotonin and noradrenaline, which are linked to mood and emotion.

On the citalopram page, the NHS lists medicines that may not mix well, including sumatriptan for migraine, strong painkillers such as tramadol and buprenorphine, anti-inflammatory painkillers such as ibuprofen and diclofenac, acid-reducing medicines such as omeprazole, anticoagulants such as aspirin and warfarin, and MAOI antidepressants such as moclobemide. It then says: do not use the herbal supplement St John's wort when taking citalopram.

The sertraline page lists anticoagulants such as warfarin and apixaban, anti-inflammatory painkillers such as aspirin and ibuprofen, and MAOIs such as moclobemide and selegiline, and gives the same instruction about St John's wort. The venlafaxine page does likewise. The wording is consistent and direct, with no suggestion of an acceptable dose or gap between them.

The NHS pages do not explain a mechanism, and this guide does not invent one. What matters in practice is that the advice is the same across three of the commonest antidepressants, and that other NHS medicine pages carry the same caution for different reasons.

The NHS lists serotonin syndrome as a possible serious side effect of citalopram, sertraline and venlafaxine, describing a fast heartbeat, sweating, shaking, twitching muscles and feeling confused or agitated. For citalopram and sertraline it calls the condition rare but serious. It is the main reason the combination of serotonin-acting medicines and supplements is treated with care.

The pages advise calling NHS 111 if you are taking the medicine and think you might be having serious side effects. If someone is very unwell, is unconscious or has a seizure, or you cannot get through promptly, use 999 or go to A&E. Do not wait to see whether symptoms settle, and do not take more of the antidepressant or the supplement.

Many people do not realise there is a problem until they read a leaflet. If you take an SSRI or venlafaxine and have been taking St John's wort, it is better to speak to a pharmacist or your prescriber than to guess. Both medicines affect how you feel, and stopping or changing either abruptly can have effects of its own.

The NHS warns that antidepressants should not be stopped suddenly, because it may cause withdrawal symptoms, and says the doctor will usually reduce the dose gradually over several weeks. That advice relates to the prescribed medicine rather than the herbal one. Tell the pharmacist exactly what product you have, the amount and how long you have used it, and bring the packet if you can.

If you are worried about low mood, a persistent change in how you feel, or thoughts of harming yourself, tell your doctor straight away. The NHS says some people have suicidal thoughts or an urge to self-harm when they start an antidepressant, that younger people or those who have had these thoughts before may be most at risk, and that you should call 999 or go to A&E immediately if you are in danger.

People often look to a herbal remedy because they want something that feels natural, or because they are waiting for an appointment. The NHS says antidepressants usually take 1 to 2 weeks to have an effect and can take up to 8 weeks to work fully, and that people who respond are usually advised to continue for at least 6 months after feeling better. That slow start can be frustrating, which is exactly when an extra product is tempting.

A GP can review what you are taking and how it is going, and the NHS notes that you will usually be seen every 1 to 2 weeks when you first start an antidepressant. If side effects are hard to manage or the medicine does not seem to help, the NHS says to talk to your doctor, who can discuss other treatment options.

Private clinicians and pharmacies can also advise, but they need the full list. The same applies to herbal practitioners: tell them the names and doses of your prescribed medicines.