Does St. John's wort affect how your medications work?

St. John's wort lowers the blood levels of dozens of common medications in ways that can have clinically serious consequences. The herb activates a metabolic system in the liver that breaks down and clears medications more rapidly. Heart transplant patients who took St. John's wort alongside ciclosporin (an immunosuppressant) experienced acute transplant rejection because the ciclosporin level in their blood dropped too low. Similar effects have been demonstrated for blood thinners, cholesterol-lowering drugs, HIV medications, sedatives and sleeping pills, and the contraceptive pill.

How strong this breakdown effect is depends on the amount of hyperforin in the product. Hyperforin is the active compound responsible for this effect. Because herbal preparations can vary considerably in composition, the degree of interaction is difficult to predict from one product to the next. Cheaper or less standardised products sometimes contain more or less than you might expect.

A separate risk arises when St. John's wort is combined with SSRI antidepressants, such as paroxetine or sertraline. That combination can cause what is known as serotonin syndrome: a dangerous condition in which you may experience agitation, confusion, muscle twitching, and an elevated heart rate. This has been reported in multiple case reports. An increased risk of side effects has also been noted with triptan-based migraine medications, although the evidence for that is less robust.

St. John's wort does genuinely work for mild to moderate depression, and that benefit is real. However, if you are also taking other medications, that benefit is difficult to weigh separately from the risks of interactions. Multiple review studies therefore conclude that people who use medications are better off avoiding St. John's wort, unless they have explicitly discussed it with their doctor or pharmacist.