Medications used to increase breast milk production increase prolactin secretion, the main hormone of lactation. There are no FDA approved medications used to improve breast feeding, but metoclopramide is used off-label and can have intolerable side effects. We examined the biological activity and safety of recombinant human prolactin (r-hPRL) as a potential medication to augment lactation. In this study, the effect of r-hPRL on breast milk production in women who did not recently deliver a baby and its effect on the bones and menstrual cycle were tested.