Nipple discharge can be caused by a wide range of conditions, most of which are harmless or easily treated.

Some of the main causes of nipple discharge are described below.

See your GP if you're unsure of the cause of your discharge or you're embarrassed or worried, especially if the discharge is bloodstained or clear and only comes from one nipple, or if you're a man with nipple discharge.

Pregnancy

For women, one of the most obvious explanations for fluid leaking from the nipple is that you're pregnant or currently breastfeeding, as this can cause a milky discharge to come from both nipples.

In pregnancy, the breasts may start to produce milk from as early as the second trimester, and some women continue to produce milk up to two years after stopping breastfeeding.

If you think you may be pregnant, you can do a pregnancy test. You may also find the article Am I pregnant? helpful.

Non-pregnancy causes

If pregnancy has been ruled out, it's likely that your nipple discharge is caused by one of the following conditions:

• duct papilloma – a harmless growth inside your breast duct (the tube that carries milk from the gland to the nipple) that typically causes bloodstained discharge from one breast
• duct ectasia – a harmless, age-related breast change that can result in a cheesy or discoloured discharge from both breasts 
• breast or nipple abscess – a painful collection of pus forming in the breast tissue or around the nipple, usually as a result of bacterial infection

These conditions are explained in more detail below, to give you a better idea of whether you may be affected. However, it's important to see your GP for a proper diagnosis so a more serious cause such as breast cancer can be ruled out.

Duct papilloma

If the discharge is bloodstained and from a single breast, the cause is likely to be a duct papilloma.

A papilloma is a harmless wart-like growth, usually about 1-2cm in size, found inside one of the breast ducts. The papilloma is usually just behind the nipple and can cause fluid or blood to seep out of the nipple. 

You'll usually just have one papilloma, and a simple procedure to remove it will often be recommended.

Although the discharge may be alarming, rest assured that papilloma is not cancer and is very unlikely to turn into cancer.

Duct ectasia

If the discharge is brown, green or cheesy, it's likely you have a harmless condition called duct ectasia.

Duct ectasia tends to affect women approaching the menopause. As the breasts age, the milk ducts behind the nipple get shorter and wider, and may produce a discharge. This is a normal, age-related change and nothing to worry about.

A lump can sometimes be felt behind the nipple, which is just scar tissue or perhaps a dilated duct, and the nipple sometimes becomes inverted.

This condition is harmless and tends to clear up without treatment. It doesn't increase your risk of developing breast cancer in the future. However, it's important to go back to your GP if you develop any new symptoms.

An abscess

If the discharge contains pus, the cause will probably be a breast abscess or abscess around the nipple. The surrounding skin will be red, warm and swollen.

Breast abscesses are often linked to mastitis – a condition that causes breast pain and swelling (inflammation) and usually affects women who are breastfeeding.

If you have been to see your GP because of mastitis, you may already have been given antibiotics. If your breast is still hard, red and painful, your GP should refer you to a specialist to check for a breast abscess.

If the diagnosis is confirmed, a simple procedure to drain the abscess may be recommended. Read more about treating breast abscesses.

More unusual causes

Less common causes of nipple discharge are:

• the contraceptive pill – discharge can be a temporary side effect of starting the pill (some women also get breast tenderness and breast enlargement)
• fluctuating hormones from puberty or the menopause
• previous breastfeeding – some women continue to produce milk up to two years after they've stopped breastfeeding  
• stimulation of the nipples – for example, through sex
• medication that causes raised levels of the milk-producing hormone prolactin – this includes SSRI antidepressants and tranquillisers
• a type of non-cancerous brain tumour called a prolactinoma, which causes raised levels of prolactin
• a hormone problem – such as an underactive thyroid gland or Cushing's syndrome
• a clogged milk duct called a galactocele – which is usually associated with childbirth and can cause a milky or creamy discharge along with a painless lump
• an early form of breast cancer called carcinoma in situ, which is found inside the milk ducts and hasn't yet spread (it's usually picked up during breast cancer screening)

Is it breast cancer?

In the vast majority of cases, nipple discharge isn't a sign of breast cancer. However, discharge from the breast can very occasionally be a symptom of cancer, so it's not something you should ever ignore.

Nipple discharge is more likely to be a symptom of cancer if:

• you also have a breast lump or changes to the skin of your breast
• the discharge is bloody and only comes from one nipple
• the discharge occurs without any pressure on your breast or nipple
• you're over 50

See your GP if you're worried you might have breast cancer. While this is unlikely, it's best to be sure by getting a proper diagnosis.

Read more about the symptoms of breast cancer and how breast cancer is diagnosed.