Prolactin is the hormone which stimulates the cells in the mother's breast to produce milk.
Domperidone increases prolactin secretion indirectly, by interfering with the action of dopamine. One of the actions of dopamine is that it decreases the secretion of prolactin by the pituitary gland. This does not mean that it cannot be prescribed for this reason, but rather that the manufacturer does not back its use for increasing milk production. It has been used, for several years, in small infants who spit up and lose weight, but it has recently been replaced for this reason by a newer drug called cisapride. Domperidone's ability to increase milk production has been recognized since it first became available.