Objective: Benzodiazepines are common therapies for mental illness and insomnia, and are used during pregnancy and lactation. Although benzodiazepines have been shown to be transferred into breast milk, the amount transferred is small and compatible with breastfeeding. However, information is not available for all drugs. Therefore, we aimed to determine the milk to plasma (M/P) ratio and relative infant dose (RID), which are used as indicators of drug transfer to breast milk, to determine the safety of such drugs for lactating women and breastfeeding infants. Methods: The study comprised of 11 pregnant women who visited the obstetrics department of Hokkaido University Hospital (approval number: 017-0131) and Tenshi Hospital (approval number: 103) for childbirth. The samples were analyzed using liquid chromatography-tandem mass spectrometry, and the M/P ratio and RID were calculated. The condition of the mother and baby at 1 month after delivery was determined from the clinical information. The target benzodiazepines were alprazolam, brotizolam, clonazepam, clotiazepam, etizolam, ethyl loflazepate, flunitrazepam, and lorazepam. Results: For all drugs, the M/P ratios were <1 and remained constant over time. For drugs other than ethyl loflazepate, the RID values were <10%, which are considered safe; however, even with ethyl loflazepate, it was only slightly >10%. No abnormalities were found in breastfeeding infants whose mothers were receiving these medications. Conclusions: The RID results of this study suggest that drug exposure through breast milk is small; thus, maternal drug treatment and breastfeeding are compatible.
Keywords: benzodiazepines; breast milk; milk/plasma ratio; plasma; relative infant dose.