Intraindividual variability and the effects of menstrual cycle phase on CYP2D6 activity were evaluated by dextromethorphan phenotyping in 20 Caucasian normal volunteers. Dextromethorphan 30 mg was administered to 10 men every 14 days for 3 months, and to 10 premenopausal women during the mid-follicular and mid-luteal phases of each menstrual cycle for three complete cycles. Urinary dextromethorphan/dextrorphan molar ratios were obtained after an overnight urine collection. Ten women and nine men were extensive metabolizer phenotypes, and one man was a poor metabolizer phenotype (confirmed by genotyping). There was no difference in dextromethorphan metabolic ratios between the mid-follicular (mean ± SD: 0.00728 ± 0.00717) and mid-luteal (0.00745 ±0.00815) phases of the menstrual cycle (P=0.88). Also, no significant difference was found in the intraindividual variability of the metabolic ratios between the two phases (P=0.80). No statistically significant sex difference in CYP2D6 activity was found between men (0.00537 ± 0.00431) and women (0.00737 ± 0.00983) extensive metabolizers (P=0.84). For all individuals, intraindividual variability in dextromethorphan ratios ranged from 12.1-136.6% with a median of 36.7%. Because hormonal fluctuations within the mid-follicular and mid-luteal phases of the menstrual cycle do not appear to affect CYP2D6 activity, pharmacokinetic or clinical investigations of CYP2D6 substrate activity may not require menstrual cycle phase stratification. Because baseline metabolic ratios may fluctuate an average of 37%, repeat baseline and treatment phenotyping assessments should be obtained for accurate determination of a given drug's effect on CYP2D6 activity when measured by dextromethorphan.