Prolactin is a hormone produced by the pituitary gland. In women, it enables the development of mammary glands, breast milk production, and its secretion maintains the cessation of menstruation as long as breastfeeding continues. Increasing the amount of prolactin in the blood, called hyperprolactinemia, may cause breast discharge and menstrual disorders until they disappear. This is normal following childbirth and during breastfeeding, which stimulates its secretion, but can sometimes be a side-effect of a localized tumor on the pituitary gland or certain medications. The hypoprolactinemia is rarer and primarily drug-induced.