Galactorrhea or galactorrhoea is the spontaneous flow of milk from the breast, unassociated with childbirth or nursing.

Galactorrhea is reported to occur in 5%-32% percent of women, much of the difference in reported incidence can be attributed to different definitions of galactorrhea. Galactorrhea also occurs in males, newborn infants and adolescents of both sexes.

Although frequently benign, it may be caused by serious underlying conditions and should be properly investigated.

It can be due to dysregulation of certain hormones or local causes such as excessive nipple stimulation. Hormonal causes most frequently associated with galactorrhea are hyperprolactinemia and thyroid conditions with elevated levels of TSH or TRH hormones. No obvious cause is found in about 50% of cases.

Lactation requires the presence of estrogen, progesterone and prolactin, and the evaluation of galactorrhoea includes eliciting a history for various medications or foods (methyldopa, opiates, antipsychotics, serotonin reuptake inhibitors, as well as licorice^{[citation needed]}) and for behavioral causes (stress, and breast and chest wall stimulation), as well as evaluation for pregnancy, pituitary adenomas (with overproduction of prolactin or compression of the pituitary stalk), and hypothyroidism. Adenomas of the anterior pituitary are most often prolactinomas. Overproduction of prolactin leads to cessation of menstrual periods and infertility, which may be a diagnostic clue. Galactorrhoea may also be caused by hormonal imbalances owing to birth control pills.