Cystitis is the medical term for inflammation of the bladder. Most of the time, the inflammation is caused by a bacterial infection, and it's called a urinary tract infection (UTI). A bladder infection can be painful and annoying, and it can become a serious health problem if the infection spreads to your kidneys. Less commonly, cystitis may occur as a reaction to certain drugs, radiation therapy, or potential irritants, such as feminine hygiene spray, spermicidal jellies, or long-term use of a catheter. Cystitis may also occur as a complication of another illness.