A mother is breast-feeding her 2-month-old infant. Which one of the following drug situations involving the mother is unlikely to cause effects in the nursing infant?

Drugs that are capable of crossing the blood-brain barrier penetrate most body tissues and can appear in the milk of the lactating mother. Though concentrations of such drugs may be low in breast milk, they may cause effects in an infant who perhaps weighs just a few kilograms. Fluoroquinolones also penetrate tissues, and because they are contraindicated in children, it seems appropriate not to risk infant exposure via breast milk. The "safest" drug situation concerns nystatin, which is used only via the topical route and, as a polyene, does not cross membrane barriers.