Discharge from the nipple can be alarming, but many normal women do have some discharge that can be expressed (squeezed) from the nipple. A 1987 study at a Boston hospital actually found 83% of women had discharge that could be expressed. These included women who were old, young, mothers, non-mothers, previously pregnant and never pregnant.
Luckily, most discharge from the breast is nothing to be alarmed about. In general, discharges that occur in both breasts at the same time are due to hormonal cause. If a woman is not breast feeding, milky discharge is known as galactorrhea. This occurs because something is increasing the prolactin levels in the body. This can be related to the pituitary gland in the brain, or other hormonal problems. Also, certain medications can increase prolactin levels in the body, including birth control pills, certain antihypertensive or blood pressure medications, or major tranquilizers.
Concern is raised about a nipple discharge when it: occurs spontaneously (without squeezing), persists, comes from a single duct, involves only one breast, and looks bloody or clear colored. If you notice any of these changes, you should call your doctor because these findings are sometimes a sign of cancer.
When nipple discharge is not bloody or clear, but rather appears yellow, green, or brown, this is less worrisome and breast cancer is considered less likely. Occasionally, the discharged liquid may be smeared on a microscopic slide or otherwise tested to evaluate for the presence of abnormal cells or blood that the naked eye cannot detect. If negative, and the discharge is present only when expressed, the patient may be instructed to refrain from expressing the discharge for several weeks. Often, the amount of discharge may decrease, or the discharge will resolve spontaneously without further treatment.
In summary, nipple discharge is a relatively common occurrence among women at certain points in their lives and usually is not worrisome unless the discharge is from one breast, from a single duct, spontaneous, and bloody or clear in color. If any of these signs are present, you should contact your physician for further evaluation.