Cetirizine is an antihistamine commonly used to treat allergic rhinitis and other allergic conditions. Cetirizine is often prescribed to breastfeeding mothers although there is limited information on infant exposure via breast milk. The aim of this study was to develop a popPK model based on data from a lactation study to predict cetirizine breast milk concentrations and estimate the relative infant dose (RID) in a breastfed infant. A popPK model was developed in NONMEM on data from a human lactation study including 35 women using cetirizine or levocetirizine while breastfeeding. Serial samples of breast milk were collected (n = 205) and the cetirizine concentrations quantified using a validated LC-MS/MS method. A one-compartment model of cetirizine in breast milk was developed and employed to calculate the relative infant dose (RID). Covariates related to the maternal characteristics and breastfeeding patterns were evaluated in the model; only milk sampling pumping duration was found to be a significant covariate, with an increasing pumping duration leading to an increased apparent milk volume of distribution (V _m ). The mean RID was 1.99% with the highest RID being 3.36% at C _max . PopPK modelling could be used to estimate infant exposure to cetirizine via breast milk. The low predicted exposure in infants supports that cetirizine is compatible with breastfeeding.