Recent studies have raised concern about serious interactions between St. John's wort and certain prescription drugs.¹ The drugs in question include cyclosporine (Neoral, Sandimmune), which is an immunosupressant given to organ transplant recipients; digoxin (Lanoxin, Lanoxicaps), which treats heart disease; theophylline (Theo-Dur), a bronchodilator used for asthma and other respiratory illness; tolbutamide (Orinase), for the treatment of diabetes; warfarin (Coumadin), a blood thinner; and indinavir (Crixivan), a protease inhibitor used in the treatment of HIV infection.

The precise mechanism of interaction is not understood, though it is theorized that St. John's wort may stimulate certain drug detoxification enzymes in the liver. The herb may also affect drug absorption. If a drug is detoxified through one of the specific pathways that St. John's wort stimulates or upregulates, the drug may be excreted quicker and have less of an effect.

Recent data show that St. John's wort reduces the plasma levels of indinavir by more than 50 percent, thought to be due to induction of the cytochrome P450 metabolic pathway.² This could potentially lead to drug resistance and treatment failure.

Other studies have suggested that St. John's wort leads to increased photosensitivity to bright light and sunburn, though this side effect was not reported in one recent clinical trial.³

As new research discloses ways in which pharmaceuticals interact with foods and dietary supplements (see "Grapefruit Juice and Drug Interactions"), the responsibility for conveying this information needs to be shared among drug and supplement manufacturers, regulatory agencies, and health care practitioners. In light of this new research, pharmacists are well advised to question patients about St. John's wort use before dispensing these prescription drugs.

References:

1. McIntyre M.
A review of the benefits, adverse events, drug interactions, and safety of St. John's wort (Hypericum perforatum): the implications with regard to the regulation of herbal medicines.
J Altern Complement Med. 2000;6(2):115-24.

2. Piscitelli SC et al.
Indinavir concentrations and St. John's wort.
Lancet 2000 Feb 12;355(9203):547-8.

3. Schulz V.
Incidence and clinical relevance of the interactions and side effects of hypericum preparations.
Perfusion 2000;13:486-90,493-4.