St John's wort has significant interactions with a large number of medicines which are in common use.
This means that it may affect the way that the body processes other medications which may make them less effective, or increase the chances of adverse effects. These medications include both prescribed drugs and medicines that you can buy for yourself over the counter.
Why might I need advice if I'm taking other medication?
If you are considering taking or are already taking St John's wort with any other prescription or over the counter medications, it is really important to get professional advice from a pharmacist or your doctor.
They will be able to talk to you about any possible interactions St John's wort may have with your medications and discuss alternative treatments or dosages.
These are some of the common drug interactions with St John’s wort:
- Antidepressants – you should not take St John's wort at the same time as any other antidepressant. Combining the two medications can make your depression worse and can lead to serotonin syndrome.
- Sleeping pills and anaesthetics – St John's wort may prolong the effects of some sleeping pills and anaesthetics. If you are taking St John's wort and are to receive a general or local anaesthetic, you should inform the anaesthetist.
- Contraceptives (including emergency contraception) – St John's wort can reduce the level of contraceptives in the blood and make them less effective, increasing the risk of pregnancy and breakthrough bleeding.
Other medications identified include antipsychotics, anticoagulants to thin the blood and immunosuppressants to prevent the body from rejecting a transplanted organ.
It can also reduce the effectiveness of common medications to treat cholesterol, heart disease, high blood pressure, migraines, epilepsy, cancer, hepatitus C, HIV and AIDS.
If you are taking St John's wort it is also best to talk to a doctor before trying light therapy for seasonal affective disorder (SAD), as St John's wort can make your skin more sensitive to light.
For a more detailed list of known medications that interact with St John's wort, you can visit the NICE website.
This information was published in January 2017. We will revise it in 2020.