croaky cat

Hi I have a 3yr old cat tom neva had problems hes a house cat. I got a new kitten a week ago which was fine xcept had fleas so I have got them both fleed at the vets. Tom was fleed wiv the spot on. Since yesterday his meow is croaky ive managed to look at his throat which is red, he has been off his food. Wood it b ok to give him some penicillin if so wat dose? Cheers

Or do I just leave him and c if he gets better? Thanks