How does galactorrhea happen?

Hormonal imbalance. Galactorrhea (breast milk production when not pregnant) can be caused by multiple events. Although our first thought is a pituitary (brain) overproduction of prolactin, frequently the cause can be be a thyroid disorder, side-effect of medication (anti-depressants, birth control pills), the result of scarring or injury to the chest wall, or even a side effect of marijuana use.