In some rare instances, a foul odor in urine can be caused by bladder cancer.
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Bladder cancer is a relatively rare condition. The symptom most commonly associated with bladder cancer is blood in the urine. The majority of people with bladder cancer will have blood or blood clots in their urine. Other symptoms of early stage bladder cancer include pain during urination, frequent urination of small amounts of urine and frequent urinary tract infections.
Bladder Cancer
As bladder cancer advances, it may cause lower back side pain and swelling in the legs.

If you have any of the signs or symptoms associated with bladder cancer, you should discuss them with your doctor. Many different conditions can cause the same symptoms as bladder cancer. You will need a doctor's help to find out what is at the source of your foul-smelling urine.