you put your cat to sleep when the quality of life he has is so low that you know he's in pain,and no amount of medicine will make him better. it sounds like he's at that point. as the lady above said, he'll understand,and be grateful that you love him so much that you're able to let him pass over the Rainbow Bridge. we had a cat that one of our neighbors found, we named him Baby Boi. he was only w/us for 3 days, but God, we were *so* in love w/him. he was so tired,he couldn't stand up, yet when we held him he purred and loved on us. he didn't eat much,and was it was in obvious pain. The vet took one look at him and said "either feline leukemia or FIV,either way, he'll have no quality of life".i said goodbye to him(my life partner was too beside herself w/grief to stay in the room), and he looked at me w/a look in his eyes that told me i was making too big a fuss, that he was ready to go.he licked my hand one last time,and nuzzled it, purring the whole time. when i knew he was ready,so was i. i won't say letting go won't hurt..it will,and tremendously so. but know this, your friend knew you loved him,and only wanted the best for him. sometimes w/animals, as w/ppl, death is a mercy. the shot is painless,and they drift off to sleep,waking up over the Rainbow Bridge, young and painfree again,ready to play, eat,and bask in the sun. you'll see him again,never to be parted. Good luck,and all my thoughts, prayers,and positive energy is yours.