Breasts can leak for a wide range of reasons, from pregnancy and breastfeeding to hormonal shifts, medications, and even friction from clothing. Most causes are completely benign, but the color, consistency, and pattern of the discharge can tell you a lot about what’s going on.
Pregnancy and Breastfeeding
The most common reason breasts leak is the body preparing for or recovering from feeding a baby. Starting around 16 weeks of pregnancy, the breasts begin producing colostrum, a thick, yellowish early milk. Some people notice wet spots on their bra or shirt well before delivery, while others never leak during pregnancy at all. Both are normal.
During breastfeeding, leaking happens because of the let-down reflex. When the nipple and the surrounding area are stimulated, nerves send a signal to the brain to release oxytocin, the hormone that pushes milk out of the breast. What makes this reflex interesting is that physical touch isn’t the only trigger. Hearing your baby cry, thinking about your baby, or even just getting ready to breastfeed can set it off, which is why many nursing parents find themselves leaking at inconvenient moments.
After weaning, it can take weeks to months for milk production to fully stop. Occasional leaking during that window is expected and not a sign of a problem.
Hormonal Causes Outside of Pregnancy
Prolactin is the hormone responsible for milk production, and it’s kept in check by dopamine, a chemical messenger from the brain that constantly tells the pituitary gland to hold back on prolactin. When that brake system is disrupted, prolactin levels climb and the breasts can start producing milk even without pregnancy or breastfeeding. The medical term for this is galactorrhea.
Several conditions can throw prolactin off balance:
- Prolactinomas. These are small, benign tumors on the pituitary gland that secrete prolactin directly. They’re the most common hormone-producing pituitary tumors.
- Hypothyroidism. When thyroid hormone levels drop, the brain compensates by releasing more of a signaling hormone called TRH. TRH doesn’t just affect the thyroid; it also stimulates prolactin production. So an underactive thyroid can indirectly cause breast leaking.
- Estrogen changes. Estrogen can raise prolactin levels by both suppressing dopamine and directly stimulating the cells that produce prolactin. This is one reason some people on hormonal birth control notice discharge.
Medications That Can Cause Leaking
A surprisingly long list of medications can trigger breast discharge by interfering with dopamine’s ability to suppress prolactin. If you’ve started a new medication and noticed leaking, it’s worth checking whether it falls into one of these categories:
- Antipsychotics and mood medications. Many psychiatric drugs, including antipsychotics, SSRIs, tricyclic antidepressants, and MAO inhibitors, block dopamine receptors in the brain.
- Anti-nausea drugs. Metoclopramide and domperidone, commonly prescribed for vomiting, are dopamine blockers and frequently raise prolactin.
- Opioids. Codeine, morphine, methadone, and heroin reduce dopamine release, which allows prolactin to rise.
- Some blood pressure medications. Verapamil, methyldopa, and reserpine have all been linked to nipple discharge through their effects on dopamine.
- Hormonal medications. Estrogen-containing birth control pills can sometimes cause leaking.
Physical Stimulation and Friction
You don’t need elevated hormones for breasts to leak. Repeated nipple stimulation, whether from a partner, tight clothing, an ill-fitting sports bra, or even a seatbelt rubbing against the chest, can activate the same nerve pathways that trigger the let-down reflex. This kind of stimulation can cause galactorrhea even when prolactin levels are only mildly elevated or completely normal. Chest wall injuries, surgeries, and even conditions like shingles affecting the chest area have also been linked to discharge through the same nerve-signaling pathway.
Mammary Duct Ectasia
This is one of the most common causes of nipple discharge in people over 45. It happens when the milk ducts beneath the nipple widen and sometimes become blocked, which is a natural part of aging. The discharge tends to be thick and can range in color from dirty white to yellow, green, or black. Many people with duct ectasia never have symptoms at all and only discover it incidentally. When symptoms do appear, they may include breast tenderness, a lump near the nipple, or a nipple that turns inward. Occasionally the blocked duct gets infected, causing redness, swelling, and fever.
What the Color Tells You
The appearance of nipple discharge is one of the most useful clues to its cause. Clear, white, yellow, green, and brown discharge can all be normal, though they can also signal a problem depending on context. Yellow discharge often points to an infection. Greenish-brown or black discharge is more typical of duct ectasia. Thin and watery discharge has different implications than thick, sticky fluid.
Bloody or pink discharge is the one color that consistently raises concern. In a study of 370 patients with watery, blood-tinged, or bloody discharge, about 13.5% had cancer, while roughly half had intraductal papillomas (small, benign growths inside the milk ducts) and about a third had fibrocystic changes.
One Side vs. Both Sides
One of the simplest ways to gauge whether discharge is likely harmless or worth investigating is whether it’s coming from one breast or both. Normal, physiologic discharge is typically bilateral (both sides), clear or milky, comes from multiple ducts, and isn’t sticky. Pathologic discharge tends to be unilateral (one side only), spontaneous rather than only appearing when you squeeze, varied in color, and often limited to a single duct opening.
Discharge that only appears when you actively squeeze or compress the breast is generally less concerning than discharge that shows up on its own and stains your clothing. Spontaneous, one-sided discharge, especially if it’s bloody, warrants evaluation.
Nipple Discharge in Men
Men can experience nipple discharge too, and the significance is quite different. In studies of men presenting with nipple discharge, 57% had an underlying malignancy, compared to about 16% of women with the same symptom. This doesn’t mean every instance of male nipple discharge is cancer. Around 43% of cases have benign explanations like duct ectasia, papillomas, or hormonal imbalances including gynecomastia. But the odds are high enough that any nipple discharge in men should be evaluated promptly.