Sapropterin is a synthetic form of the naturally occurring enzyme cofactor tetrahydrobiopterin (BH4) and is used in the treatment of phenylketonuria. BH4 is found in normal human milk and is a cofactor in multiple reactions including serving as a catalyst to phenylalanine hydroxylase. In two postmarketing pregnancy registries of women taking sapropterin, a total of 16 women were identified as breastfeeding for a mean of 3.5 months. No lactation-related safety concerns were reported in infants of mothers nursing during maternal treatment with sapropterin. United States and European guidelines state that sapropterin dihydrochloride supplementation is not contraindicated as an adjunct to dietary therapy in breastfeeding women who are responsive to BH4.[1,2]