[Sashy]

If your pet is sick/not eating/fever, DON'T overload his system with another toxin. Get them well first before using leave on anti-flea medication because whatever it is, anti-fleas are poisons after all. Especially leave on treatments, make sure your animal is healthy as there will be some absorption through the skin. If you have a bad flea infestation use a topical treatment and rinse it off well, then pick/comb the weakened fleas.