Most infections are thought to occur when bacteria that live harmlessly in the bowel or on the skin get into the bladder through the urethra (the tube that carries urine out of your body) and start to multiply.
Cystitis is much more common in women than men, probably because the anus (back passage) is closer to the urethra in women and the urethra is much shorter.
It's not always obvious how the bacteria get into the bladder.
But some things can increase the risk of it happening, including:
What can increase your risk?
There are a number of things that can increase your chances of developing an infection in your bladder.
Some of these are outlined on this page.
Not being able to empty your bladder
If you're unable to empty your bladder fully, any bacteria that get inside may not be flushed out when you go to the toilet and can multiply more easily.
You may not be able to empty your bladder fully if:
- you have a blockage in your urinary system, such as a bladder stone
- you're pregnant, as the baby may be pressing on your bladder
- (in men) you have an enlarged prostate gland that presses on the urethra
For women who have been through the menopause or are going through it, the lining of the urethra can shrink and become thinner because of a lack of the hormone oestrogen.
The natural balance of bacteria in the vagina may also change, which can allow potentially harmful bacteria to become more common.
This can make the urethra more vulnerable to infection, which could spread into the bladder.
You're more likely to get cystitis if you have diabetes, a condition where the level of sugar in your body becomes too high.
High levels of sugar in your urine can provide a good environment for bacteria to multiply, so any bacteria that get into the bladder are more likely to cause cystitis.
Other causes of cystitis
Cystitis can also be caused by damage or irritation to the urethra and bladder.
This can be the result of:
- friction from sex
- chemical irritants, such as those in perfumed soap or bubble bath
- damage caused by a catheter or surgery on your bladder
- radiotherapy to your pelvis or treatment with certain chemotherapy medicines
- a woman's genitals having been deliberately cut or changed for cultural, religious and social reasons (an illegal practice called female genital mutilation, or FGM)
Cystitis has also been linked to recreational use of the drug ketamine.