Prolactin is the hormone that 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. However, there are several studies which show that it works to increase milk production and that it is safe. Cisapride does not have the effect on milk production which domperidone has.