The nipples can release a small amount of fluid. Most causes are benign.
The key factors in discharge are age, one side v. both sides, and conditions. The content of the discharge is important but the appearance of the fluid varies.
People of any age may have discharge related to an illness. Illnesses that cause a discharge are rare in people less than 40 (2%), but common for people older than 60 (30%).
Discharge from both nipples is normally benign. The color of the fluid is normally not that helpful clinically- it can be clear, milky, yellow, gray, brown or green. Benign discharge can be caused by changes in the hormones, medications, or stimulation of the nipple.
Discharge from one nipple that is spontaneous (no stimulation) and persistent tends to be important. Many cases are related to a condition called intraductal papilloma. This is a nonmalignant condition, although some medical providers recommend to remove this. A malignant condition causes unilateral discharge in a small fraction of cases.
Mammograms and ultrasound are helpful tests to evaluate nipple discharge, when it is suspicious for a problem. The fluid’s blood content matters, this can be tested regardless of its color. An abnormal mammogram, ultrasound, or blood test indicates the need for a more thorough evaluation.