lethargic depressed cat

My cat Ian is really really lethargic still and has been two days
since he came back from having his teeth cleaned and a couple
extracted from the vet. He seemed ok at first, eating as soon as he
was back and purring when petted etc,,,now he just lies very still -
occasionally wanders somewhere in the house - and then collapses with
his head on the floor looking depressed.

He is hardly touching his food now, and I am unable to give him the
anti-biotics because he won't eat.

Is this quite typical cat behaviour? Should I be worried?