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Standardized extracts from the Ginkgo biloba tree are purported to exert positive neurocognitive effects and may also be useful in the treatment of a variety of vascular and other disorders. This dietary supplement is among the most commonly used herbal preparations in the world. The objective of this study was to assess in normal volunteers (n= 12) the influence of standardized Ginkgo biloba (GB) on the activity of cytochrome P-450 (CYP) 2D6 and 3A4 normal volunteers phenotyped as CYP2D6 extensive metabolizers. Probe substrates dextromethorphan (CYP2D6 activity) and alprazolam (CYP 3A4 activity) were co-administered orally at baseline, and following treatment with GB (120 mg twice daily) for 14 days. Urinary concentrations of dextromethorphan and dextrorphan were quantified and dextromethorphan metabolic ratios (DMRs) were determined at baseline and after GB treatment. Likewise, plasma …