Eleutherococcus senticosus (Siberian ginseng) is a commonly used herbal preparation. The objective of this study was to assess in normal volunteers the influence of a standardised E. senticosus extract on the activity of cytochrome P450 CYP2D6 and 3A4. Probe substrates dextromethorphan (CYP2D6 activity) and alprazolam (CYP3A4 activity) were administered orally at baseline and again following treatment with E. senticosus (1 × 485 mg twice daily) for 14 days. Urinary concentrations of dextromethorphan and dextorphan were quantified, and dextromethorphan metabolic ratios (DMRs) were determined at baseline and after E. senticosus treatment. Similarly, plasma samples were collected (0–60 h) for alprazolam pharmacokinetics at baseline and after E. senticosus treatment to assess effects on CYP3A4 activity. Validated high-performance liquid chromatography methods were used to quantify all compounds and relevant metabolites. There were no statistically significant differences between pre- and post- E. senticosus treatment DMRs indicating a lack of effect on CYP2D6. For alprazolam there also were no significant differences in the pharmacokinetic parameters determined by non-compartmental modelling, indicating that E. senticosus does not significantly induce or inhibit CYP3A4. Our results indicate that standardised extracts of E. senticosus at generally recommended doses for over-the-counter use are unlikely to alter the disposition of co-administered medications primarily dependent on the CYP2D6 or CYP3A4 pathways for elimination.