Impact on Therapeutic Drug Monitoring

Amitava Dasgupta, PhD


1. Introduction

2. Regulatory Issues Affecting Herbal Medicines

3. Drug-Herb Interactions

5. Interaction of Warfarin with Herbal Supplements

6. Interaction of Garlic (Allium Sativum) with Drugs

7. Interactions of Ginseng with Drugs

8. Interactions of Ginkgo with Drugs

9. Interaction of Kava with Drugs

10. Food-Drug Interactions

11. Conclusions

Interactions between herbal supplements and drugs and between food and drugs are of significant clinical importance. St. John's wort, a popular herbal supplement, induces cytochrome P 450 mixed function oxidase as well as modulated P-glycoprotein in intestine and reduces plasma concentrations of cyclosporine, tacrolimus, amitriptyline, digoxin, fexofenadine, indinavir, methadone, midazolam, nevirapine, phenoprocoumon, squinavir, simvastatin, theophylline and warfarin. These reductions may cause treatment failures. Interaction of warfarin with herbal supplements may increase or decrease the anticoagulant effect of warfarin. Important interactions of various drugs with ginseng, ginkgo biloba, kava and garlic have also been reported. Consumption of a single glass of grapefruit juice caused a twofold to threefold increase in the plasma concentration of felodipine, and pharmocokinetics of approximately 40 other drugs are also affected by intake of grapefruit juice. In contrast, bioavailability of fexofinadine was significantly reduced by grapefruit juice. Orange, cranberry, pomelo and pomegranate juice also may alter


From: Handbook of Drug Monitoring Methods Edited by: A. Dasgupta © Humana Press Inc., Totowa, NJ

bioavailability of certain drugs. Interactions between piperine, a constituent of black pepper and certain drugs, have also been reported.

Key Words: Bioavailability; drug; food; herbal supplements; interactions; pharmacokinetics.

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