Hartz 3 in 1

I put a hartz 3 in 1 flea collar on my 8 1/2 month old cat and within an hour he threw up, then went and curled up in a ball on my bed for 2 hours. Usually Marshall is really active and he’s never thrown up before so I knew something must be wrong and removed the flea collar. That was 3 days ago and he has barely eaten since! I’ve tried buying him some wet food with no luck and have tried hand feeding him dry food but he only eats a few bites and then goes and lays in his kitty tunnel. I feel so bad I can’t believe they are selling a product that’s so dangerous to cats!