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Objective: Venlafaxine use during pregnancy has increased over the past decade in concert with accumulating reproductive safety data; however, systematic data on venlafaxine during lactation remain sparse. The current study characterizes the level and determinants of venlafaxine and desvenlafaxine concentrations in breast milk and in nursing infant plasma., Method: Women participating in a prospective investigation of perinatal pharmacokinetics from January 2001 through July 2006 who were treated with venlafaxine and who chose to continue venlafaxine during lactation were included in the analysis. Breast milk samples were collected via breast pump from foremilk to hindmilk from a single breast to determine the excretion gradient, and serial samples were collected over 24 hours to determine the time course of excretion. Paired maternal/infant plasma samples were also collected. Venlafaxine and desvenlafaxine concentrations were determined using high-performance liquid chromatography with ultraviolet detection. Statistical analyses of breast milk and infant plasma concentrations and their determinants were conducted., Results: Thirteen women and their nursing infants participated, providing 106 breast milk samples. The mean milk/plasma ratio was 275.3% (95% CI = 144.8% to 405.7%). There were statistically significant time courses of excretion for venlafaxine (R = 0.36, F = 6.82, P < .02), desvenlafaxine (R = 0.48, F = 4.41, P < .009), and combined venlafaxine/desvenlafaxine (R = 0.51, F = 5.16, P < .004), with the highest venlafaxine and desvenlafaxine concentrations in the breast milk occurring 8 hours after maternal ingestion. Infant plasma concentrations for combined venlafaxine/desvenlafaxine were 37.1% of maternal plasma concentrations. The theoretical infant venlafaxine/desvenlafaxine dose was 0.208 mg/kg/d, and the relative infant venlafaxine/desvenlafaxine dose was 8.1%. The theoretical and relative infant doses for desvenlafaxine were 197% and 224% higher, respectively, than those for venlafaxine. No adverse events were observed or reported in the nursing infants., Conclusions: Consistent with previous investigations of medications in breast milk, the venlafaxine and desvenlafaxine milk/plasma ratios were highly variable. The rate of venlafaxine/desvenlafaxine excretion into human breast milk is relatively higher than that observed for other antidepressants, largely due to higher desvenlafaxine excretion. These data expand the extant literature on venlafaxine and desvenlafaxine in lactation., (Copyright 2009 Physicians Postgraduate Press, Inc.)