Cyclophosphamide, together with its active metabolites, aldophosphamide, acrolein and nitrogenated mustard, is an antineoplastic from the group of nitrogen mustards with very marked immunosuppressive properties. Cyclophosphamide is excreted in breast milk (Wiernik 1971). In two studies, breastfed infants developed neutropenia, thrombocytopenia, and anemia. (Durodola 1979, Amato 1977). Given the variability in interindividual pharmacokinetics, potential pharmacokinetic changes with co-administration with other medication and its serious side effects, it is prudent not to breastfeed during treatment (Anderson 2016, Götestam 2016, Grunewald 2015, Pistilli 2013, Østensen 2006, WHO 2002, Pediatrics 2001). When possible, detection in breast 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 before returning to breastfeeding (Anderson 2016). Based on this, expert authors recommend waiting 72 hours after the last dose (practically 10 T½) to restart breastfeeding. Meanwhile, express and discard breast milk regularly (Hale 2017 p.239).