<b>Introduction</b>: The purpose of this study is to elucidate the influence of drugs on infants and to provide information about safe drug treatments during breastfeeding using the FDA’s Adverse Event Reporting System (FAERS).<br><b>Study Design</b>: Case series based on FAERS data.<br><b>Methods: </b>We used the cleaned FAERS data in JAPIC AERS, extracted cases of adverse events from the category “neonatal exposure through breastfeeding (2000189)” in Standardized MedDRA Queries, and collected “the cases of breast-feeding infants” by system organ class, the type of adverse event, therapeutic category of first suspected drugs, and the generic name of the drugs.  We aggregated the data of the most reported cases of suspected drugs from first to fifth by the name of the adverse event.  Additionally, we investigated the properties and disposition of each suspected drug and verified the transitivity of breast milk as well.<br><b>Results: </b>Only 551 cases of breast-feeding infants (0.01%) were extracted from the data of JAPIC AERS.  The aggregated data of adverse events in terms of system organ class showed high numbers of gastrointestinal disease (such as diarrhea and vomiting) and impairment of the nervous system (such as drowsiness), and drugs which acted on the nervous system were the most suspected drugs.  There were 26 cases of adverse events associated with lamotrigine, which was the most frequently reported.<br><b>Discussion: </b>We could observe the characteristics of adverse events and suspected drugs that were shown as the influence of drugs taken during breastfeeding which were exposed to infants.  Our study showed 2 conclusions: 1) the drugs that have had adverse events frequently reported have the characteristic of facilitating the drug’s migration into breast milk, and 2) the most frequently reported cases were those in which nursing mothers or medical experts could recognize the correlation between breast milk and the adverse event(s) immediately after breast milk was given to the infant.