Breast milk concentrations of acetaminophen and diclofenac - unexpectedly high mammary transfer of the general-purpose drug acetaminophen

Background Breastfeeding is considered to be the most effective way of ensuring the health and survival of newborns. However, mammary transfer of drugs administered to mothers to breastfeeding infants remains a pressing concern. Acetaminophen and diclofenac sodium are widely prescribed analgesics for postpartum pain relief, but there have been few recent reports on the mammary transfer of these drugs, despite advances in analytic techniques. Methods We conducted a study on 20 postpartum mothers from August 2019–March 2020. Blood and milk samples from participants were analyzed using liquid chromatography-electrospray ionization tandem mass spectrometry within 24 hours after oral administration of acetaminophen and diclofenac sodium. The area under the concentration-time curve (AUC) was calculated from the concentration curve obtained by a naive pooled-data approach. Results For acetaminophen, AUC was 36,053 ng/mL.h and 37,768 ng/mL.h in plasma and breast milk, respectively, with a milk-to-plasma drug concentration ratio of 1.048. For diclofenac, the AUC was 0.227 ng/mL.h and 0.021 ng/mL.h, in plasma and breast milk, respectively, with a milk-to-plasma drug concentration ratio of 0.093. Conclusions While diclofenac sodium showed low mammary transfer, acetaminophen showed a relatively high milk-to-plasma drug concentration ratio. Given recent studies suggesting potential connections between acetaminophen use during pregnancy and risks to developmental prognosis in children, we believe that adequate information regarding the fact that acetaminophen is easily transferred to breast milk should be provided to mothers.