St John's wort

Most supplements are interesting for what they do. This one is interesting for what it does to everything else.

The mechanism, in one paragraph

St John’s wort induces the liver enzymes and transporters that clear many medicines from the body. Induction means the body makes more of them, so the drug is broken down faster.

The consequence is not a side effect in the usual sense. It is that another medicine stops working properly at its normal dose, quietly, without anything obviously going wrong. A drug that underperforms produces no symptom of its own; it produces the return of whatever it was preventing.

That is why this substance appears on interaction lists more often than almost anything else sold without a prescription, and why the affected list includes categories where underperformance is serious.

What the guidance says

NHS information for warfarin is direct: “Do not take the herbal remedy St John’s wort or vitamin K supplements while you’re taking warfarin.”

The same page makes the general point that applies far beyond warfarin: before taking it, “tell your doctor if you’re taking any other medicines, herbal remedies, vitamins or supplements”.

The phrase “herbal remedies” sitting in that list alongside medicines is the useful part. Herbal is not a separate category from pharmacology; it is pharmacology with less labelling.

Why the packaging tells you none of this

A supplement label lists ingredients and amounts. It carries no interactions section, because that is not part of the format.

So the product on the shelf that most needs a warning is the one whose category is least equipped to carry one. Someone buying it for low mood, from a shop rather than a pharmacy, has no prompt to think about their other prescriptions at all.

This is the sharpest example of the argument on the page about reading labels when you take a prescription, and it is the reason that page exists.

The names to recognise

Hypericum perforatum is the botanical name and it appears on labels that avoid the English one. Hypericum alone is common. In combination products, especially sleep and mood formulas, it can sit in the middle of a list of botanicals where nobody is reading closely.

Standardised extracts are often described by their hyperforin or hypericin content, which is the constituent most associated with the enzyme induction. A higher standardised extract is not a milder product.

The other direction

Grapefruit is the mirror image and worth knowing alongside it. Grapefruit inhibits the enzyme that clears certain drugs, so the same dose reaches a higher level. St John’s wort induces it, so the same dose reaches a lower one.

One makes a medicine too strong and the other makes it too weak, by acting on the same machinery in opposite directions. Neither is on any warning label, and both are sold in ordinary shops.