Cats, especially male cats frequently suffer from blockage of the urethra. This is a very serious emergency. A cat can rupture their bladder if completely blocked for long enough or do severe kidney damage.
What to watch for:
You may have noticed them going to the litter tray more often then usual but only urinating in small amounts, there may be blood in the urine, your cat may be drinking more.
When they completely block they are seen to be straining in their litter tray and producing no urine, crying while in litter tray, appear distressed, go off food.
If they start to vomit they are becoming toxic with nitrogen waste and can rupture their bladder within hours.
If you notice any of these symptoms, please call the vet.