According to the Mayo Clinic, lactation outside of pregnancy or breastfeeding, or galactorrhea, is usually caused by an abnormal excess of the hormone prolactin, which normally stimulates milk production. This, in turn, can have several causes, including drug use, certain herbs, pituitary tumors, hypothyroidism or excessive breast stimulation. Galactorrhea can also manifest without any known cause or in response to injuries of the breast or spinal cord.
The Mayo Clinic states that galactorrhea is not, in itself, a cause for alarm. This is particularly the case if it is accompanied by the use of birth control pills, prescribed opiates, antidepressants, antipsychotics or blood pressure controlling drugs. If hormone levels are not raised, it may just be due to the variability of sensitivity to prolactin in different individuals.
According to the Mayo Clinic, even those who are unable to become pregnant, including men and newborns, can sometimes experience abnormal milk production. In men, it's a sign of testosterone insufficiency, especially if it's accompanied by breast enlargement, tenderness and a decrease in libido. In newborns, this is typically a result of the connection between mother and child. While in the womb, the newborn is exposed to high levels of maternal estrogen, which can cause some breast development and milk production.