St John's wort

What it is

St John's wort is a yellow-flowered plant used in European herbal medicine for centuries. Extracts are sold as mood supplements and, in some countries, as licensed herbal remedies for mild low mood.

It is one of the most important supplements to know about from a safety point of view, because it changes how the body handles many prescription drugs. Natural does not mean harmless here.

How it works

It affects several brain chemicals, including serotonin, in a way similar to some antidepressants. It also switches on liver enzymes and transport proteins (including CYP3A4 and P-glycoprotein) that clear many medicines from the body faster, making them less effective.

What the evidence says

• Moderate Mild to moderate depression
  Trials and reviews show benefit over placebo for mild to moderate low mood, comparable to some antidepressants in some studies, but it is not suitable for severe depression.
• Limited Menopausal symptoms and mood
  Some small studies, often in combination products.
• Weak Anxiety
  Evidence is inconsistent.
• Limited Seasonal low mood
  Small studies only.

Evidence ratings are our plain-English summary of how much good human research exists, not a promise of results. How we rate evidence.

Dosage and timing

• Follow the label of a registered product exactly; dosing varies by extract.
• Typical studies use 900 mg of standardised extract a day, split into doses.
• Do not start or stop it suddenly if you take other medicines. Speak to a pharmacist or GP first.
• In the UK, look for products carrying a Traditional Herbal Registration (THR) mark.

Forms and what to look for

• Registered traditional herbal medicines (THR) are the most reliable UK option, as they follow quality and labelling rules.
• Standardised extracts list hypericin and/or hyperforin content.
• Teas and loose herb are inconsistent in strength.
• Products without clear standardisation or registration are best avoided.

What to expect, and when

• Mood effects take around 2-6 weeks to appear, if at all.
• Enzyme induction starts within about 1-2 weeks and fades over about 1-2 weeks after stopping.
• If you have not noticed improvement after 4-6 weeks, speak to your GP about other options.

Side effects

• Dry mouth, upset stomach or dizziness.
• Photosensitivity: skin may burn more easily in sunlight.
• Fatigue, restlessness or vivid dreams.
• Headache.
• Serotonin syndrome if combined with antidepressants (agitation, sweating, tremor, fast heartbeat).

Interactions

• Combined hormonal contraception (the pill, patch, ring): St John's wort can make it less effective and cause breakthrough bleeding. Unplanned pregnancy has been reported. Use extra contraception or avoid it.
• SSRIs, SNRIs, MAOIs, triptans and other serotonergic drugs: risk of serotonin syndrome.
• Warfarin and some other anticoagulants: reduced effect, and risk of clots.
• Ciclosporin and tacrolimus (transplant drugs): reduced levels, with risk of organ rejection.
• HIV medicines, some cancer drugs, digoxin, some statins, some epilepsy medicines and many others: reduced effectiveness.
• Always check with a pharmacist before taking it alongside any prescription.

Who should avoid it

• Anyone taking antidepressants or other prescription medicines, unless cleared by a clinician.
• Women using hormonal contraception.
• People with bipolar disorder (risk of triggering mania) or psychosis.
• Pregnancy and breastfeeding.
• Anyone with severe depression or thoughts of self-harm: see a GP or call NHS 111 or Samaritans on 116 123.
• People with fair skin who burn easily in sun, and before surgery (stop in advance, as advised).

Risks and safety

• Reduced effectiveness of many critical medicines is the main risk, and it can be serious.
• Serotonin syndrome when combined with antidepressants is potentially dangerous.
• It is not for self-treating significant depression.
• Be careful with unregistered products, which can vary widely in strength.

Buying well: quality and testing

• In the UK, choose products with a THR mark.
• Look for a standardised extract with clear mg per tablet.
• Avoid mixed blends that obscure the dose.
• Keep the box and tell any clinician you see that you take it.

Bottom line

St John's wort has genuine effects, which is exactly why it is risky. If you take any prescription medicine, or use hormonal contraception, check with a pharmacist before touching it. For low mood that lasts more than a couple of weeks, a conversation with a GP is a better starting point than a herbal supplement.