If your cat is urinating or defecating outside the litter box, we certainly understand your frustration. Luckily, this is a treatable condition in most cases. A medical exam, as well as a few simple changes can help to re-establish proper litter box use.
The first step includes taking your cat to your veterinarian. Whenever a cat suddenly eliminates outside of the litter box, it's strongly advised to get a physical exam including urine analysis and in some cases blood work in order to rule out any illness or injury that may be causing the behavior. Once a medical reason for the lapse in box use has been ruled out, you want to consider behavioral reasons.
There are two main behavioral reasons for failure to eliminate in an established litter box. One is marking, which is a form of communication. The second one is inappropriate elimination, which is a toileting behavior. Both behaviors may occur for a variety of reasons. In some cases this is not as easy as it seems to identify the reason and you will need a professional to help you.
This document created by the San Francisco SPCA with a grant from Maddie's Fund®