Some causes of a cat not using a litter box include behavioral problems, medical problems, a dirty litter box, changes in the type of litter used and too much litter in the litter box, according to WebMD. Cats who are no longer using the litter box need a veterinary exam.
A veterinary exam can confirm or rule out medical problems that may be causing the litter box problems. Medical problems that cause a cat to stop using its litter box include bladder stones, feline interstitial cystitis, urinary tract infections and a blockage in the urinary area, according to WebMD. If the veterinary exam shows no medical reason for the cat's litter box issues, then it is recommended that the pet owner look at behavioral problems and at the litter box itself to address potential issues, notes the Larimer Humane Society.