There are currently no common guidelines used by health professionals to aid decision-making around the use of medicines during breastfeeding. Several specialized books, Web sites, and drug information services exist; however, all use slightly different criteria to derive their "safety hierarchy." The aim of this study was to build consensus among international experts in pharmacology and breastfeeding to develop an agreed-upon classification system for safety of medicine use in breastfeeding women.A three-round Web-based Delphi qualitative research design was used.Seventeen experts in pharmacy/pharmacology and breastfeeding identified 15 key parameters that are used to inform decisions about medicines and breastfeeding. The most important parameters about the infant were the age and health of the child, and those of the medicine were the safety profile and experience of use in infants. The experts had a clear understanding of the complexity of decision-making when prescribing for breastfeeding women. Although the current number or letter classification systems do not incorporate important considerations such as infant age, a longer "descriptive text" incorporating all considerations may be impractical in busy clinical practice.Although clinicians and lay people would appreciate a simple classification scheme, in practice, decision-making about the safety of medicines for breastfeeding women is complex.