Permethrin is a pyrethroid, not an organo phosphate. It is toxic to cats, fish and aquatic animals but safe for most mammals. It can occaionally have neuro-toxic side effects similar to organo phosphates, but is not absorbed through the skin like organo phosphates are (that's why its toxic to cats, all that licking gets it into their stomach). And of course Bayer say for prevention of ticks, no guarentee, it can be rendered less effective by excessive swimming or bathing, they don't want to be having to defend themselves all the time. They don't state what they can't prove. It's not perfect, but I don't know of anything more effective. My dog hasn't had a deer tick bite as long as I remember to use it regularly.