The packaging, however, may be a critical factor. John Gicking, an emergency and critical care vet at BluePearl Veterinary Partners Tampa hospital, isn't familiar with this specific collar. But, he tells The Dodo, "it is known that some flea products for dogs are toxic for cats."
While that's not the culprit in Onyx's case - the Sergeant's collar Crandall bought is specifically for cats - Gicking suggests talking to a vet about flea and tick remedies before buying something at the pet store.
Erika Loftin, a critical care specialist with DoveLewis Emergency Animal Hospital, doesn't recommend flea collars at all.
"They tend to have a very local affect," she tells The Dodo. "In other words, they might control the fleas on the head or the neck but if some of the fleas come running down the animal and bite them some place else."
Tragically, it's also possible for a pet to die from the wrong treatment.
"I think, in general, the way they work is there are chemicals associated with the collar, embedded in the collar, that are toxic to fleas," she explains. "Fleas are more sensitive to that than mammals because they have differences in their nervous systems.
"But I think there is always a chance that you'll have a particularly sensitive animal or perhaps a collar that was intended for one type of animal put onto another type of animal."
But there were other warnings. Specifically, hundreds of messages about the collar on Consumer Affairs, a site dedicated to product testimonials.
Disturbing accounts extend even to Amazon, where the product's comment section is riddled with warnings - from one customer complaining about "severe blistering" around a cat's neck to another claiming the collar induced painful vomiting.
Of course, these users may be no more legit than the ones who rave on the same site about how it "works well on my 5 cats!"
(There is, unsurprisingly, nary a negative comment on Sergeant's own Facebook page, which boasts more than half a million likes.)