CHAPTER 275 Handbook of Developmental Neurotoxicology. http://dx.doi.org/10.1016/B978-0-12-809405-1.00024-9 Copyright © 2018 Elsevier Inc. All rights reserved. 24 Drug and Chemical Contaminants in Breast Milk: Effects on Neurodevelopment of the Nursing Infant Tom Leibson, Prateek Lala, Shinya Ito Hospital for Sick Children, Toronto, ON, Canada I. INTRODUCTION Breastfeeding is associated with tangible benefits for the mother and infant, including lower infection rates and higher cognitive function in the infant. Beneficial effects of breast milk on infant neurocognitive develop- ment are particularly intriguing. 1,2 Although the effects of confounding factors cannot be ruled out, or will nev- er be ruled out, there is no reason to believe that infant formula is superior to breast milk under most circum- stances. Besides nutritional factors, milk apparently con- tains molecular information of the surrounding habitat (including food the mother ingests), which is communi- cated to the infant through the ingested milk, modifying the offspring’s immune function for later life. 3 Indeed, the current pediatric practice guidelines invariably advocate exclusive breastfeeding for at least the first 6 months of life. 4 However, the presumed role of the mammary gland as an information-processing organ of the surrounding environment also indicates that nonnutrient substances such as drugs the mother takes are excreted into milk. Importantly, more than half of breastfeeding women take medications, 5 highlighting the magnitude of the OUTLINE I. Introduction 275 II. Effects of Breastfeeding on Infant Neurodevelopment 276 III. Exposure of Nursing Infants to Drugs in Breast Milk 277 A. Mammary Gland Structure 277 B. Mechanisms of Xenobiotics Secretion Into Milk 277 1. Ionization Characteristics 277 2. Lipid Solubility 277 3. Plasma Protein Binding 277 C. Quantitative Estimation of Infant Drug Exposure Via Breast Milk 277 IV. Effects of Maternal Exposure to Environmental Contaminants on the Nursing Infant 278 A. Metals and Inorganic Compounds 278 1. Lead 278 2. Methlymercury 278 3. Arsenic 279 B. Persistent Organic Pollutants (POP) 279 1. Polychlorinated Dibenzodioxins (Dioxins) and Polychlorinated Dibenzofurans (Furans) 279 2. Polychlorinated Biphenyls 279 V. Drugs and Nonmedicinal Substances 280 A. Marijuana (Cannabis) 280 B. Other Nonmedicinal and Medicinal Substances 280 VI. Information Resources 280 VII. Conclusions 280 References 283