The medical term for lactation outside of pregnancy and childbirth is galactorrhea. It has been observed in males(1)(2). In newborns, it is relativity common in both sexes, and is called "witch's milk."(3) Prolactin is the hormone associated with lactation and it is secreted by the anterior pituitary gland.(4)
The causes are varied. According to the American Academy of Family Physicians some causes are:
- Medicines, like hormones, antidepressants and blood pressure medicines
- Herbs, such as nettle, fennel, blessed thistle, anise and fenugreek seed
- Street drugs, like marijuana and opiates
- Hormone imbalance
- Tumors (usually benign), especially tumors of the pituitary gland (say: pit-too-it-tarry), which is in the brain
- Clothing that irritates the breasts (like scratchy wool shirts or bras that don't fit well)
- Doing very frequent breast self-exams (daily exams)
- Stimulation of the breast during sexual activity
Sometimes the cause can't be found.
(1)Kleinberg, David L., Gordon L. Noel, and Andrew G. Frantz. "Galactorrhea: A Study of 235 Cases, Including 48 with Pituitary Tumors." New England Journal of Medicine (1977).
(2)Finn, James E., and Lester A. Mount. "Galactorrhea in Males with Tumors in the Region of the Pituitary Gland." Journal of Neurosurgery 35.6 (1971): 723-27.
(3)Madlon-Kay, DJ. "'Witch's Milk'. Galactorrhea in the Newborn." American Journal of Diseases of Children (1986).
(4)PEÑA, KRISTIN S., and JO ANN ROSENFELD. "Evaluation and Treatment of Galactorrhea." American Family Physicians (2001).