Doxorubicin is a cytotoxic anthracycline antibiotic isolated from cultures of the caesium variety of Streptomyces peucetius. Doxorubicin and its active metabolite doxorubicinol are excreted in breast milk in significant amounts and with a very high milk/plasma ratio (Pistilli 2013, Egan 1985). There are two distinct pharmaceutical forms of doxorubicin, in the form of hydrochloride and in liposomal form, with very different pharmacokinetic profiles (Gabizon 2003). Given the variability in interindividual pharmacokinetics, potential pharmacokinetic changes with co-administration with other medication (EMA 2017, Swenson 2003) and their serious side effects (cardiotoxicity, myelotoxicity and liver toxicity) (Tacar 2013, Danesi 2002), it is prudent not to breastfeed during treatment. When possible, detection in the milk of each patient to determine the total elimination of the drug would be the best indicator for resuming breastfeeding between two rounds of chemotherapy. It is known via pharmacokinetics that after 3 elimination half-lives (T½) 87.5% of the drug is eliminated from the body; after 4 T½ 94%, after 5 T½ 96.9%, after 6 T½ 98.4% and after 7 T½ 99%. Plasma drug concentrations in the body are negligible after 7 T½. In general, a period of five half-lives may be considered a safe waiting period to return to breastfeeding (Anderson 2016). For doxorubicin in hydrochloride form, with a mean elimination half-life (T½) of 30 hours, authors recommend waiting 7 to 10 days (between 5, 6 and 8 T½) after the last dose to restart breastfeeding. Meanwhile, express and discard breast milk regularly (Hale 2017 p.210). For the liposomal form with a mean T ½ of 74 hours and a wide range, this is not applicable. Some chemotherapeutics with antibiotic effects may alter the composition of the microbiota (combination of bacteria or bacterial flora) of the milk and the concentration of some of its components (Urbaniak 2014). This possibly occurs briefly with later recovery, with no harmful effects being reported in breastfed infants.