According to Scientific American, men are able to lactate, but only under extreme circumstances. Starvation and pituitary gland stimulation are two methods of causing lactation in men.
The Other Side of Science reports that male lactation is called galactorrhea and can signal an underlying health problem for the man. Male lactation is often due to a surge of the hormone prolactin which can come about as a result of malnutrition, liver disease, a hormonal imbalance or a pituitary tumor. The use of recreational drugs or pathogens in the body can sometimes produce a surge in prolactin production, as can puberty on a temporary basis.