Galactorrhea (also spelled galactorrhoea) (galacto- + -rrhea) or lactorrhea (lacto- + -rrhea) is the spontaneous flow of milk from the breast, unassociated with childbirth or nursing.

[1] Lactation requires the presence of prolactin, and the evaluation of galactorrhea includes eliciting a history for various medications or foods (methyldopa, opioids, antipsychotics, serotonin reuptake inhibitors[4]) and for behavioral causes (stress, breast, and chest wall stimulation), as well as evaluation for gestation, pituitary adenomas (with overproduction of prolactin or compression of the pituitary stalk), and hypothyroidism.

Overproduction of prolactin leads to cessation of menstrual periods and infertility, which may be a diagnostic clue.

Galactorrhea may also be caused by hormonal imbalances owing to birth control pills.

Galactorrhea is also a side effect associated with the use of the second-generation H2 receptor antagonist cimetidine (Tagamet).