There was a really good reply over at another site and I'm going to post it here:
By all means have the pet examined to rule out underlying medical diseases. But keep this in mind..Medication is NEVER the answer to behavior problems. The answer is behavior modification. If the cat will not use the box for urination and defecation then you have a litter aversion. There is something wrong with the box, the litter, it's location etc. Either it is in the wrong place,( too noisy) or the box is the wrong type ( covered vs uncovered), or there aren't enough boxes ( 1 box per cat in the house is the rule), or it's not being cleaned correctly ( using Lysol and other disinfectants) repels cats.
If all possible litter box concerns have been ruled out then one may be dealing also with a marking behavior. The young one is maturing and may be challenging the status quo for his place in the hierarchy. there's no way I can tell with the scant history provided.
The cat needs a COMPLETE behavior work up so that the proper behavior modification protocols can be formulated. If your vet doesn't have the time/interest/expertise then get a referral to one who does. Medication is ONLY indicated in specific circumstances. For example if the problem is anxiety related and the level of anxiety is such that the cat cannot learn the behavior modification protocols. Too many vets and owners rely on drugs as a quick bandage. There ARE no quick fixes with behavior problems. You have to have a LOT of patience, dedication and willingness to take the time it requires to solve the problem. Often it'll require trying many different approaches before finding one that works.
I suggest you read the article on Inappropriate Elimination in the Library. that'll give you some basic information. then search the forum because this subject has been covered MANY times. In addition these sites provide good legitimate information:http://www.veterinarypartner.com/Con...?P=C&C=240&S=2http://www.catsinternational.org/index.htmlhttp://web.vet.cornell.edu/Public/FHC/Housesoiling.htmlhttp://www.animalbehavior.org/Applie...sesoiling.htmlhttp://www3.us.elsevierhealth.com/co...tructions.html
Margaret Muns DVM