St. John's wort does not interfere with therapeutic drug monitoring of 12 commonly monitored drugs using immunoassays.

St. John's wort, a popular herbal remedy for depression, is known to interact with many Western drugs because of the ability of its components to induce liver enzymes. Lower concentrations of various drugs due to increased clearance have been reported. Because immunoassays are commonly used in clinical laboratories for therapeutic drug monitoring, we studied the potential interference of St. John's wort with commonly monitored therapeutic drugs. Drug-free serum pools were supplemented with St. John's wort to achieve in vitro St. John's wort concentrations mimicking in vivo concentrations after both recommended use and overdose. Concentrations of digoxin, tricyclic antidepressants (TCAs), phenytoin, carbamazepine, theophylline, valproic acid, quinidine, phenobarbital, procainamide, and N-acetyl procainamide were measured in serum. Pooled serum specimens from patients who were taking a particular drug were also supplemented in vitro with concentrations of St. John's wort to investigate whether observed concentrations changed after supplementation with St. John's wort. The effect of St. John's wort on cyclosporine and tacrolimus (FK 506) was studied in whole blood. We found no significant interference from St. John's wort with any assay studied. Moreover, when drug-free serum was supplemented with very high concentrations of hypericin (2 microg/mL) and hyperforin (2 microg/mL) pure standard, we observed no apparent drug level with any immunoassay. The presence of both hypericin and hyperforin was also confirmed by thin layer chromatography (TLC) in both preparations of St. John's wort. We conclude that immunoassays may be used to measure levels of therapeutic drugs in patients who self-medicate with St. John's wort.