St. John's wort
Meaning of St. John's wort: A yellow-flowered plant sold as a supplement, best known in medicine for how strongly it interferes with other drugs.
Definition of St. John's wort
St. John's wort is a yellow-flowered plant, Hypericum perforatum, sold as a herbal supplement and taken most often for low mood. Its extracts contain hypericin and hyperforin, the compounds usually credited with its activity. In medicine it is best known for speeding up the liver enzymes that break down other medicines, which lowers the amount of those drugs in the blood.
What is in the plant?
The flowering tops are dried and extracted. Two constituents receive most attention: hypericin, a red pigment, and hyperforin, which appears to account for much of the effect on mood. Products vary widely, because the amount of each compound depends on the plant, the harvest and the extraction method. Two bottles labeled the same way can differ considerably in strength, a general feature of herbal preparations rather than a fault unique to this one.
Why do the drug interactions matter?
St. John's wort switches on a liver enzyme system called cytochrome P450, and a transport protein that pumps drugs back out of the gut. Both changes clear other medicines faster, so blood levels fall and the medicine may stop working. It is one of the few supplements with interactions well enough documented to appear in standard drug references.
| Affected medicine | Documented effect |
|---|---|
| Combined oral contraceptives | Reduced levels, with breakthrough bleeding reported |
| Ciclosporin and tacrolimus | Lower levels, with transplant rejection reported |
| Warfarin | Reduced anticoagulant effect |
| Some antidepressants | Additive serotonin effects |
What is it taken for?
Mild to moderate depression is the common use, and it is a prescription medicine for that purpose in some countries while remaining a supplement in others. Trials have produced mixed results, with stronger findings in milder illness. A recognized effect is increased sensitivity of the skin to sunlight.
Also known as
Used in a sentence
The medication review noted that the patient had been taking St. John's wort alongside a prescribed anticoagulant.
Good to know
St. John's wort induces cytochrome P450 enzymes and P-glycoprotein, which lowers blood levels of many prescription medicines including contraceptives, ciclosporin and warfarin.