Most milky discharge is caused by lactation or increased mechanical stimulation of the nipple due to fondling, suckling or irritation from clothing during exercise or activity. Prolactin is the hormone produced by the pituitary gland that starts the growth of the mammary glands and triggers production of milk. Opalescent discharge that is yellow or green in color is normal. Papillomas frequently involve the large milk ducts near the nipple. Multiple papillomas may also be found in the small breast ducts further from the nipple.
Most opalescent discharge is due to duct ectasia or cyst.
Papilloma usually causes discharge from a single breast duct. Both male and female adolescents may experience a milky discharge during puberty. Nipple discharge in the adult male is more often associated with a malignant condition than in the female. Mammography should be performed and biopsy should be performed if a mass or mammographic abnormality is found. During pregnancy and lactation, breast tissue grows rapidly and this can lead to duct irritation that causes bloody nipple discharge.