Based on plasma and milk concentrations, a PopPK model will be performed (i) to document the average and individual exposure to amoxicillin in milk, (ii) to identify the factors explaining the inter-individual pharmacokinetic variability and (iii) to determine the daily dose ingested by the infant and its variability for different dosage regimens applied to the mother. The secondary objectives are to monitor (i) predefined adverse reactions in infants and (ii) the impact on milk production.

This study will complement the clinical data on amoxicillin in breastfed new-borns considering that current knowledge in terms of authorized drugs use during breastfeeding is not sufficient to guarantee the child absence of exposure, even minimal. As a consequence, this lack of certainties leads to premature discontinuation of breastfeeding, well known to be beneficial for the breastfed new-borns. Amoxicillin, prescribed alone or with clavulanic acid at 2 to 3g per day for 7 days, including in lactating women, is mainly used to treat β-hemolytic strep throat, infections of the genitourinary tract as well as ear, nose and throat infections.

Inclusion Criteria:

• Mothers over 18 years, treated by amoxicillin for at least 2 days who breastfeed their child, - Participation requires that breastfeeding is already well established (i.e., at least 2 weeks after giving birth in order to sample mature milk) and that the infant is over 4 weeks old,
• Affiliation to the social security scheme or equivalent,
• Ability to understand and willingness to sign a written Informed Consent document.

Exclusion Criteria:

• Mothers under 18 years old,
• Infants born prematurely (i.e., gestational age under 34 weeks) or requiring appropriate medical supervision or support from a team of social workers,
• Mothers who gave birth to twins,
• Inability to communicate due to language problems for the mother,
• Patient subject to a legal protection order (curatorship or tutorship).