St John’s wort (Hypericum perforatum) is a yellow-flowering plant used for centuries as a herbal medicine, most often for mild low mood. In the UK, it’s sold as capsules, tablets, teas and tinctures, and it’s one of the few herbal products with real clinical evidence behind its use.
That same activity is why it needs care. St John’s wort affects the same brain chemistry as prescribed antidepressants, and it’s one of the most interaction-prone supplements you can buy. It changes how your body processes a long list of common medicines, from the contraceptive pill to blood thinners, which is where “natural” stops being a reassurance.
St John’s wort is best known for treating mild to moderate depression. People often turn to it as a herbal option for anxiety and depression, but its one well-evidenced use is depression: multiple clinical trials and reviews have found it can work about as well as standard antidepressants for mild to moderate symptoms, with fewer reported side effects. It’s prescribed for this in some countries, but in the UK it’s sold as a food supplement rather than a licensed medicine.
Its other uses are far less certain. People also take it for symptoms of the menopause, anxiety, poor sleep and premenstrual symptoms or apply it to the skin for minor wounds and inflammation. The evidence for these is thin and rests largely on tradition rather than clinical trials.
St John’s wort contains several active compounds, mainly hypericin and hyperforin. These are thought to raise levels of brain chemicals, including serotonin, dopamine and noradrenaline, by slowing their reuptake, broadly the same mechanism many antidepressants use. That’s why it can ease mild low mood, and why combining it with an antidepressant is risky.
Hyperforin also ramps up a liver enzyme system (cytochrome P450) and a transport protein that clears drugs from cells. These handle a large share of prescription medicines, so when St John’s wort speeds them up, other drugs can be cleared too quickly and stop working properly.
Between them, these two effects, shifting brain chemistry and speeding up drug clearance, are what drive nearly every St John’s wort interaction.
In controlled trials, a standardised extract eased the symptoms of depression, improving mood, motivation and day-to-day functioning about as well as standard antidepressants, often with fewer side effects such as nausea, drowsiness or reduced sex drive.
A large Cochrane review of 29 trials found St John’s wort worked better than placebo and about as well as standard antidepressants for mild to moderate symptoms, with fewer people stopping treatment because of side effects. It isn’t suitable for severe depression, which needs a proper clinical assessment.
Some women use it for menopausal symptoms such as hot flushes, irritability and low mood. There’s limited trial evidence that it may help, mainly with the mood side of menopause, but the studies are small, and the effect is far less certain than in depression.
These uses come mostly from traditional medicine rather than modern trials. St John’s wort has long been taken for anxiety, restlessness and poor sleep, and applied to the skin for minor wounds, burns and inflammation. There’s anecdotal support and some early research, but not the trials that back its use for low mood.
St John’s wort is very well known for its drug interactions, and it works in two directions. It speeds up the clearance of some medicines so they drop below the level they need to work, and it pushes others, especially those affecting serotonin, too far the other way. A supplement that looks harmless can quietly weaken or dangerously amplify a prescribed treatment.
The medicines below are among the most important to know about, but it isn’t a complete list. If you take anything regularly, check with a pharmacist before starting.
St John’s wort can reduce the effectiveness of hormonal contraception, including the combined pill, the progestogen-only pill and some other hormonal methods. By clearing the hormones faster, it can lower them enough to cause breakthrough bleeding or, more seriously, unintended pregnancy. If you rely on hormonal contraception, this is one of the clearest reasons not to start it without advice first.
Taking St John’s wort with prescribed antidepressants, particularly SSRIs (selective serotonin reuptake inhibitors) is a combination to avoid outright. Because both raise serotonin, combining them can trigger serotonin syndrome, a potentially serious reaction. Its physical symptoms include:
Never combine the two without speaking to your GP or pharmacist first.
St John’s wort affects a wide range of prescription medicines. Some of the most common ones are:
| Medicine | Why it matters |
| Warfarin and other anticoagulants | Reduced effect, raising clot risk |
| Epilepsy medicines | Lower levels can mean less seizure control |
| Some HIV treatments | Reduced effectiveness |
| Immunosuppressants (after transplants) | Lower levels risk organ rejection |
| Certain heart medicines | Reduced effect |
| Some cancer treatments | Reduced effectiveness |
This isn’t a complete list, and the risk depends on your exact medicines and doses. A pharmacist can check whether St John’s wort is safe alongside your prescription before you start.
There’s no well-documented direct interaction between St John’s wort and GLP-1 injections like Mounjaro or Wegovy. That doesn’t make the pairing automatically safe: these medicines can already affect how well oral contraception is absorbed, and St John’s wort reduces contraceptive effectiveness by a different route.
Before combining St John’s wort with any weight loss treatment, check with a pharmacist who can review everything you’re taking.
For a healthy adult taking nothing else, St John’s wort is generally well tolerated, and its side effects tend to be mild. Its side effects and interactions matter far more when you’re taking other medicines, so the more useful question is: safe for whom, and when?
The most common side effects are:
Most settle down, but the increased sun sensitivity is worth taking seriously if you have fair skin or spend a lot of time outdoors.
Some people should avoid it altogether:
When it comes to evaluating the safety of St John’s wort, it’s your circumstances and other medicines, far more than the herb itself, that decide whether it’s right for you.
If you’ve weighed it up and still want to try St John’s wort, a few steps make it far safer.
Speak to a pharmacist, GP or other healthcare professional first. They can check it against your medicines and history. Be upfront about everything you take, including prescriptions, over-the-counter remedies, and other supplements, as these are most likely to interact.
Don’t start or stop suddenly if you’re on another treatment. Adding it to an existing medicine or dropping a prescribed treatment to switch to it can cause problems on its own. Plan any change with a professional.
Buy from a reputable source. Herbal supplements aren’t regulated as tightly as licensed medicines, so strength and quality vary. A recognised brand or a pharmacy gives you more confidence in what you’re taking, and stick to the dosage on the label unless a pharmacist tells you otherwise.
If you’re already a Click2Pharmacy patient, or considering one of our treatments, a pharmacist can check whether St John’s wort fits with your medicines as part of your consultation.
St John’s wort can help with mild low mood, but only when it’s safe for you and the medicines you already take.
If you’re taking a prescription treatment from Click2Pharmacy, or thinking about starting one, our pharmacists will review the medicines you take as part of your consultation, and flag anything St John’s wort might interfere with.
St John’s wort isn’t a recognised cause of weight gain, and some people report the opposite. Low mood itself can affect appetite and weight in either direction, making any change hard to blame this as the cause. If you’re worried about weight changes on any treatment, it’s worth speaking to a pharmacist.
St John’s wort usually takes a few weeks to have a noticeable effect on mood, often around four to six weeks, rather than working straight away. Because of that delay, it’s common for people to assume it isn’t working and stop too early.
Yes, St John’s wort is sold over the counter in the UK as a herbal supplement, in pharmacies, health shops and online. Being easy to buy doesn’t mean it’s automatically right or safe for you, especially if you take other medicines. A quick check with a pharmacist is the best way to know whether it suits your situation.
It’s generally better to taper St John’s wort off rather than stop dead, especially if you’ve been taking it for a while. Stopping suddenly can bring back the symptoms you were managing, and it can cause problems if you’re switching to a prescribed antidepressant. Speak to a pharmacist or GP about the safest way to stop, particularly if a change of treatment is involved.
