My cat, Savino, is 16 years old and a year ago was 19 pounds. He is now 7.5 pounds. He throws up a lot and his stool is always diarrhea. Sometimes he eats nonstop but recently I find he is eating very little and not using the litter box as much. I haven't taken him to the vet yet however, I asked my vet and they say possibly cancer or kidney problems. What do you think?
Your cat should have been seen by a vet at the first sign of weight loss or vomiting or diarrhea. There are several conditions that older cats get that are very treatable if caught early. It's so much better for the cat and far less expensive for you if you seek help right away.
There are a couple of other diseases that I would put on the list of possibilities ahead of kidney failure and cancer.
Cats with kidney failure do not urinate less. They drink excessively and urinate excessively and don't have great appetites ever.
The top two differentials on my list would be diabetes or hyperthyroidism, both of which are very treatable if diagnosed in time. It may be too late. That amount of weight loss is SO serious.
At any rate, a vet visit is still in order. Maybe it's not too late. Even if it is, your cat is suffering and needs to be evaluated and you need to have options presented to you.
Please don't hesitate to make that appointment or go to the emergency room.