One of the most obvious explanations for fluid leaking from the nipple is that you're pregnant or currently breastfeeding.
Nipple discharge may also be caused by a wide range of conditions, most of which are harmless or easily treated.
If you are unsure of the cause of your breast discharge or you're embarrassed or worried, see your GP, especially if the discharge is bloodstained or only comes from one breast.
Am I pregnant?
If the discharge is milky and comes from both nipples, the most likely explanation is that you're pregnant.
In pregnancy, the breasts may start to produce milk from as early as the second trimester (weeks 14-26), and some women continue to produce milk up to two years after stopping breastfeeding.
If you think you may be pregnant, see your GP or pharmacist for a pregnancy test. You can also read our article on Am I pregnant?
If pregnancy has been ruled out, it's likely that your nipple discharge is caused by one of the following conditions:
duct papilloma – this is 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 – this is a harmless, age-related breast change that can result in a cheesy or discoloured discharge from both breasts
breast or nipple abscess – this is a painful collection of pus that forms 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 professional diagnosis and treatment.
If the discharge is bloodstained and from a single breast, the cause is likely to be duct papilloma.
A papilloma is a harmless wart-like growth, usually about 1-2cm in size, found inside one of the breast ducts. The breast ducts carry milk from the gland to the nipple (see diagram). 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, which means it can be easily removed.
Although the discharge may be alarming, rest assured that papilloma is not cancer and is not likely to turn into cancer. However, you should still get it checked by your GP so they can rule out breast cancer and offer you any necessary treatment.
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, and the nipple sometimes becomes inverted.
Even though this condition is harmless and tends to clear up without treatment, it's important that you see your GP so they can rule out breast cancer. Having duct ectasia does not increase your risk of developing breast cancer in the future.
If the discharge contains pus, the cause will probably be a breast abscess or abscess around the nipple.
An abscess is a painful collection of pus that usually forms under the skin after a bacterial infection. The surrounding skin may also be red, warm and swollen.
A breast abscess is usually a complication of mastitis (inflammation of the breast). If you have been to see your GP because of mastitis, you may already have been given antibiotics. If after taking antibiotics your breast is still hard, red and painful, your GP may refer you to a specialist breast unit to confirm the diagnosis of a breast abscess.
More unusual causes
Less common causes of nipple discharge are:
the contraceptive pill – the discharge is usually just 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 disease
a clogged milk duct called a galactocele, which is usually associated with childbirth
an early form of breast cancer called carcinoma in situ, which is found inside the milk ducts and hasn't yet spread (it is usually picked up during a routine mammogram)
When you should be referred
Your GP should refer you to a specialist for further investigation if:
you're over 50 (unless there's an obvious harmless reason for your discharge)
you have a bloody discharge
you are a man with nipple discharge
Breast cancer is an unlikely cause, but needs to be ruled out, especially if:
you are over 40
you have a breast lump
your discharge contains blood
only one breast is affected