Two goals when counseling breastfeeding mothers taking medication are protecting the infant from adverse events and permitting necessary maternal therapy. Madadi et al. report a case-control study of neonatal and maternal opioid toxicity after codeine administration. Therapeutic considerations in counseling breastfeeding mothers include susceptibility to drug toxicity of the very young and/or premature infant, significant interindividual variations in drug response, the dose-response relationship with respect to drug toxicity, and the role of pharmacogenetics in both the mother and the infant. These host factors may combine in a particular patient to act synergistically to produce an adverse reaction.