It is not always what we think it is

Have you noticed that your cat is suddenly spending his time going into and out of his litter box? Or that he stays there for a long time without managing to relieve himself?
He may be constipated, but the more likely cause of this behaviour is a urinary tract obstruction, which requires an emergency visit.
Act quickly; if there is a complete obstruction, the animal could die in a very short period of time.