OBJECTIVE: To quantify the relative infant dose of quetiapine during breast feeding, describe the milk:plasma(M:P)-ratio-: and determine the well-being of the exposed infant.CASE SUMMARY: A 26-year-old mother and her 3-month-old son were studied over a 24 hour quetiapine dose interval at steady-state. Quetiapine concentrations were quantified by high-performance liquid chromatography. Infant exposure was calculated as the concentration in milk multiplied by an estimated milk production of 0.15 L/kg/day and normalized,to the weight, adjusted maternal dose. The average concentration in milk was 41 mu g/L, the M:P ratio (measured using average concentrations in the elimination phase) was 0.29, and the relative infant dose was 0.09% of the maternal weight-adjusted dose (7273 mu g/kg/day). The infant plasma concentration of 1.4 mu g/L was some 6% of the corresponding maternal plasma concentration. No adverse effects were noted in the infant.DISCUSSION: Our findings of an Want exposure to quetiapine of less than 0.1% of the maternal dose and a lack of adverse effects confirm and extend the findings of 2 previous studies.CONCLUSIONS: Although limited, the-data shown here support the prescription of quetiapine to a breast-feeding mother following a careful individual risk/benefit analysis. We suggest regular monitoring of infant progress and occasional measurement of quetiapine in the infant's plasma.