My boy was 15.  He died this morning at 9.10am.
Getting old (and fat) he wasn't getting about much but, certainly, didn't appear to be in any pain.  He appeared to have a couple of warts on his tail so I took him to the vets to check it out. 
The vet said that he wasn't grooming as he was artheritic and could not reach.  She prescribed Metacam - the 5.5 marker on the dropper?daily.
We went for check ins through the first and second bottle and all was well.  Until March when he was stumbling around the house, very disorientated, and then could not manage to walk at all. He was panting like a dog.
It being a Sunday, I took him to the 24 hour vet who gave him a diferent pain killer(don't know what - I've always trusted the vet).  He did not appear to have improved and I took him to my regular vet on the Monday.  They kept him in for blood tests which she assured me were fine (for his age) although I questioned his  LFTs (they had been high last year when I had his teeth cleaned).  She said they were a little high but nothing to worry about.  She said he may be starting to suffer from dementia and gave me some Diasapam for emergencies.
He appeared to pick up and was his usual naughty self til Monday when, again, his mobility was poor and his breathing laboured.  He was sitting with his weight bearing on to his front paws.  I thought he must be having a 'flare up' of his arthertis.  I took him to the vet again yesterday who said his kidneys did not feel 'right' and he was concerned about his respiratory system.  I said that his last blood tests had showed his renal function was okay but he said, no, it had been 'high' and the Metacam could damage his kidneys.  I found your site last night - far, far too late.
We went this morning for xrays but, apparently, he had trouble breathing whilst they were trying to take the blood tests and died.
The vet said he probably had a lot of problems, kidney, heart ect.  I don't understand why they continued prescribing the Metacam if they were aware his kidneys were (had become) a problem and he had no previous history of any ill health.
I may just be looking for a 'reason' but I feel that I have poisoned my little boy.