Ms. A, a 27-year-old woman, was admitted to a psychiatric hospital 3 weeks after delivery of her daughter. She was suffering from a severe depressive episode with suicidal thoughts. Ms. A had a history of a first depressive episode at age 18 that was not treated. At the time of admission to the mother-child unit, Ms. A was breast-feeding her child and had so far not received antidepressive treatment. A routine diagnostic assessment, including a physical examination, laboratory studies, and a cerebral computerized tomography scan were normal. Treatment with sertraline, 150 mg/day for 11 weeks, was not effective. Therefore, Ms. A was switched to mirtazapine, 30 mg/day at 9:00 p.m. She fed her infant six times a day. Concentrations of mirtazapine were determined in breast milk and in the serum of mother and infant by using mass spectrometry after Ms. A provided written informed consent and the study was approved by the local ethics committee for measurement of her and her infant’s serum levels of the drug. Samples were taken after reaching steady state before breast-feeding, the first time at 7:00 p.m. (22 hours postdose) and a second time at 12:00 a.m. (15 hours postdose).

At 7:00 p.m., the maternal plasma level of mirtazapine was 7 ng/ml (therapeutic range=5–100 ng/ml); the same level was found in the foremilk (the early portion), whereas in the hindmilk (the later portion), a concentration of 18 ng/ml was detected. On the next day at 12:00 a.m., the maternal plasma concentration was 25 ng/ml; in the foremilk, a concentration of 28 ng/ml and in the hindmilk, 34 ng/ml were found. The infant’s plasma concentration was 0.2 ng/ml. The body weight of the infant was 6.8 kg at this time.

Ms. A was discharged in remission after 6 weeks of mirtazapine treatment. The psychomotor development of the infant was normal, as rated by an experienced neuropediatrician. No adverse events related to the mother’s mirtazapine intake could be detected; especially, there was no sedation or abnormal weight gain.