[xkcd-sucks]

The systemic bug killers are fucking terrifying. They're not approved for human use, so there's some baseline concern. They're neurotoxins which are "specific" to bugs [0], but, all living things are remarkably similar and no drug is 100% specific to its target. Finally, it's formulated to persist in a body for a month, which means it's really hard to flush out an overdose!!!!!!

The product booklet in revolution / frontline rx is actually really comprehensive and worth a read here, it goes in to the pharmacology in detail.

... In spite of all this I still do poison my cat's bugs but try to pad the dosing schedule by a week or 2 each time, it's not like the life cycle of the critical parasites is longer than that.

[0] mostly glutamate gated chloride channel activators, and mammals don't have these receptors. but mammals do have glutamate gated cation channels (AMPAs/NMDAr) and the drugs bind activate them too, just only a little tiny bit... that adds up in an overdose. What you get from excitatory glutamate channel activation is seizures.