Background <p>Amoxicillin is frequently prescribed to peripartum women as prophylaxis and treatment for infection. Despite frequent usage of amoxicillin in lactating women, quantitative evidence regarding its transfer between systemic circulation and breastmilk remains limited. However, such data is fundamentally essential for assessing the safety risk of medicine usage during breastfeeding.</p> Objective <p>This research performed population pharmacokinetic (PopPK) modeling and simulation to characterize the pharmacokinetics of amoxicillin in plasma and breastmilk in lactating Göttingen Minipigs, a physiologically relevant animal model to humans that enables studying medication transfer during lactation.</p> Methods <p>Demographic characteristics and amoxicillin concentrations in plasma and breastmilk following daily intramuscular doses (7&#xa0;mg/kg) in 9 minipigs from two studies were included in the PopPK analysis. The milk-to-plasma (M/P) ratio, daily infant dosage (DID), and relative infant dose (RID) were calculated based on the simulated steady-state exposure.</p> Results <p>The final model consisted of a two-compartment distribution with zero-order absorption and linear elimination. The central compartment was deemed as the plasma compartment with time-varying volume of distribution, and linked to the breastmilk compartment via bidirectional first-order transfer. No covariate was included. The developed PopPK model well described amoxicillin in plasma and breastmilk in the lactating Göttingen Minipigs. The predicted M/P ratio, DID, and RID of amoxicillin following the maximal doses were approximately 0.25, 0.11&#xa0;mg·kg⁻¹·day⁻¹, and 1.6% for Göttingen Minipigs, respectively.</p> Conclusions <p>The model-based simulation indicated minimal amoxicillin exposure to the breastfed piglets, suggesting a low safety risk, which was reinforced by the absence of adverse events in the piglets.</p> Graphical Abstract <p></p>