I put a flea treatment on him yesterday morning, and he's still lethargic and barely eating now, 36 hours later. He's 18 months old, but has only been with us a few months... This is the first treatment we've put on him. Googling has me concerned it's an allergic reaction 
.
Obviously I'll ring the vet in the morning if he's still not right, but how worried should I be?