Both women and men can produce breast milk if they have or are given certain hormones such as oxytocin and prolactin. Women who have just given birth will usually continue to produce breast milk as long as they continue to nurse. In rare cases, women and men have been able to produce breast milk without the influence of medication.
A tumor on the pituitary gland may cause hyperprolactinemia (excess prolactin in the blood) which will cause you to produce breast milk so if you are producing milk and are not pregnant or nursing you should see a doctor.