Frontline--active ingredient fipronil in all Frontlines®, plus s-methoprene in Frontline Combo--is a commonly used, popular treatment sold by veterinarians for control of adult fleas and flea larvae in cats. It's usually free of side effects, but in rare cases toxicity has been reported.
Toxicity signs are drooling, vomiting and hair loss, redness and itching at the site of product application. It's very unusual for anything more serious to happen unless you inadvertently overdose your cat. See your veterinarian urgently if symptoms are prolonged or worrying. Front line's two ingredients have slightly different toxicity risks.
Toxicity of S-Methoprene
S-methoprene (11.8 per cent of a single dose) carries a slight acute toxicity risk, "may be" a carcinogen--a cancer-causing agent--and can disrupt the animal's endocrine system--the system of glands that regulate hormones in a cat's body.
Toxicity of Fipronil
Fipronil (9.8 per cent of single dose) carries moderate acute toxicity risk, is more likely to be carcinogenic and it's "suspected" of disruption to the endocrine system.
The Frontline data sheet indicates that it should not be used on kittens under 8 weeks, or weighing less than 1kg. No special precautions are noted in use on pregnant and lactating cats, but it should not be used on debilitated animals.
Use of the "spot-on" version avoids overdose. Follow the instructions. Use in-date products and apply between the shoulders where kitty can't lick. Using overlapping flea products increases overdose risk.