Breastfeeding is known to be associated with higher cognitive function in the infants that may be observed years after breastfeeding. Of many factors that determine magnitude of drug excretion into milk, plasma protein binding, ionization characteristics and lipophilicity of drug are the most important. Low plasma protein binding, cationic nature, and higher lipophilicity are often associated with higher degree of drug excretion into milk. Milk-to-maternal plasma ratio of drug concentrations (MP) ratio is estimated from milk and plasma AUC (area under the curve), and expressed as an average parameter over a time period. Risk assessment of drug therapy during lactation is characterized by several factors that are distinct from those of pregnancy. Breastfeeding-related information is available for the chemotherapeutic agents in cancer-treatment settings, but levels of evidence are not high enough to make firm recommendations.