Aims To investigate the transfer of escitalopram and its demethyl metabolite into milk, the absolute and relative infant doses via milk and to assess any unwanted effects in the breastfed infant.Methods Multiple samples of blood and milk were obtained over a dose interval at steady state from eight women who were taking escitalopram for postnatal depression. Drug concentrations in plasma and milk were measured by high-performance liquid chromatography and milk/plasma ratio (M/P-AUC), absolute infant dose and relative infant dose were estimated by standard methods. Their breastfed infants were also examined clinically and in five infants a blood sample was taken for drug analysis.Results The median dose taken by the women was 10 mg day(-1). The mean (95% confidence interval) M/P-AUC was 2.2 (2.0, 2.4) for escitalopram and 2.2 (1.9, 2.5) for demethylescitalopram. Absolute infant doses were 7.6 mu g kg(-1) day(-1) (5.2, 10.0) for escitalopram and 3.0 mu g kg(-1) day(-1) (2.4, 3.6) for demethylescitalopram. The total relative infant dose for escitalopram plus its demethyl metabolite was 5.3% (4.2, 6.4) as escitalopram equivalents. All of the infants had met normal developmental milestones and no adverse effects were seen. Compared with average maternal plasma concentrations (24 mu g l(-1)), the concentrations of the parent drug and its metabolite in plasma from five infants were most commonly below the limit of detection (<= 3 mu g l(-1)).Conclusion The study shows that escitalopram is safe for use during breastfeeding. Because its absolute infant dose is lower than that for an equivalent antidepressant dose of rac-citalopram, it may be preferred over rac-citalopram in treating depression in lactating women. Nevertheless, each decision to breastfeed should always be made on the basis of an individual risk:benefit analysis.