PURPOSE:To examine differences in longer-term breastfeeding outcomes among mothers of preterm infants according to domperidone exposure status, as well as examine the potential for effect modification according to maternal weight. METHODS:Retrospective cohort study of 198 mothers of very preterm infants (born ≤ 30 weeks' gestation) who initiated breastfeeding and whose infants survived until hospital discharge. Data on domperidone use were obtained from hospital pharmacy records, with the primary outcome defined as continuation of breastfeeding at infant discharge from the Neonatal Unit. The relationship between domperidone exposure and breastfeeding status was investigated using multivariable regression analysis, adjusting for potential confounders. Additional pre-determined analyses were undertaken following stratification according to maternal weight to investigate the presence of effect modification. RESULTS:No overall difference was observed in the proportion of mothers continuing to breastfeed at the time of infant discharge from the Neonatal Unit according to whether or not they received domperidone (aRR 0.99; 0.86-1.13). Notably, effect modification was observed according to maternal weight, with use of domperidone associated with a reduced likelihood of breastfeeding at discharge among women ≥ 70 kg (aRR 0.72; 0.54-0.97), but not among those < 70 kg (aRR 1.16; 0.92-1.46). CONCLUSIONS:Despite experiencing low milk supply, longer-term breastfeeding outcomes were similar between women who did and did not use domperidone. Differences in domperidone effectiveness according to maternal weight have important implications for clinical practice given the increasing prevalence of overweight/obesity in reproductive-age women and their higher risk of low milk supply, highlighting the importance of further research in this area.