Cancer patients who take the herbal remedy St John's wort (Hypericum perforatum) may jeopardise their response to chemotherapy, according to a small study that examined the effect of the herb on the metabolism of the antineoplastic agent irinotecan (Journal of the National Cancer Institute 2002;94:1247-9).

St John's Wort is known to induce the cytochrome P450 hepatic enzyme system. Specifically, the herb induces the expression of an isoform of the P450 enzyme system known as CYP3A4. This enzymatic cascade is heavily used by the body to break down a variety of drugs and toxins.

More than half of the chemotherapeutic agents now used are broken down …