Health Canada sounds warning over St. John's wort

Health Canada is warning physicians that a popular herbal remedy for depression could "seriously alter or diminish the effectiveness" of prescription medications.

A study conducted by the National Institutes of Health in Maryland found that St. John's wort (Hypericum perforatum) significantly decreased the presence of indinavir, a protease inhibitor used to treat HIV infection, in the bloodstream. The study found that St. John's wort decreased plasma concentrations of indinavir, possibly by inducing the cytochrome P450 metabolic pathway.

Although the study (Lancet 2000;355:547-8) referred only to indinavir, officials at both the US Food and Drug Administration and Health Canada are urging patients taking St. John's wort concurrently with any retroviral drug to consult their physician. In a letter to physicians, Health Canada also warns that St. John's wort may affect the metabolism of a variety of prescription drugs, including anti-epilepsy agents, oral contraceptives and immunosuppressant, antidepressant and anticoagulant drugs.

"Although drug interaction studies with St. John's wort are not available for antiretroviral agents other than indinavir, the possibility of significant decreases in the blood concentrations of all of the currently marketed HIV-1 protease inhibitors should be considered," warned the letter.

Patients have also developed "serotonin syndrome" by using St. John's wort in combination with conventional antidepressants. The syndrome is characterized by tremors, headaches and restlessness.