Nicotine transfers into breast milk, with concentrations in smokers' milk being three times higher than maternal plasma levels, and infants eliminate nicotine much slower than adults.

“nicotine transfers into breast milk too and can have harmful effects on infant health nicotine concentrations in breast milk of women who smoke are three times higher than the mom's plasma levels but an infant's capacity to eliminate nicotine is three to four times less than that of the adult”