Colistimethate has been assigned to pregnancy category C by the FDA. In animal studies, colistimethate was teratogenic and embryotoxic in rabbits, but not teratogenic in rats. There are no controlled data in human pregnancy. Colistimethate crosses the placenta and should only be given during pregnancy when benefit outweighs risk.

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Colistimethate is excreted into human milk in small amounts (milk-to-plasma ratio 0.17 to 0.18). The possibility of bowel flora modification and interference with culture results should be considered. The manufacturer recommends that caution be used when administering colistimethate to breast-feeding women.

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