Chemicals

Exposures to environmental chemicals, such as DDT, dioxin, and methyl mercury
Hepatitis C
Illicit drug use, such as amphetamines, cocaine, heroin, and marijuana
Implants and breast surgery
Metabolic disorders such as galactosemia, a condition in which the infant cannot metabolize lactose, a sugar found in all mammalian milk
Tobacco and alcohol use, since alcohol and nicotine are present in breast milk. However, for women who cannot or will not stop smoking, breast-feeding is still advised, since the benefits of breast milk outweigh the risks from nicotine exposure