By now, those of you with close ties to the veterinary profession may already know: Comfortis (spinosad) and whopping doses of ivermectin do not go hand in hand.

Comfortis is the new-generation, super-effective, oral flea killer I’ve been using on my most flea-allergic and flea-infested patients. At this point I’ve got quite a cache of Miami-area canines sucking spinosad fumes with apparent gusto (as in, Comfortis works…it works really well).

Perhaps that’s why I received a flurry of emails and faxes on Friday (June 26th, 2008) informing me of the news: DO NOT USE COMFORTIS ON DOGS RECEIVING IVERMECTIN!

Of course this raised an alarm, not least because I’d already been informed of the potential for negative interaction of these insecticides (spinosad and ivermectin)—and I’d been assured of Comfortis’s safety when administered with the labeled doses of ivermectin veterinarians currently use on a monthly basis for heartworm prevention (in Heartgard, Iverhart, etc.).

So I checked my emails, shuffled the faxes and quickly came to the conclusion that the FDA’s warning related to the large, extra-label doses of ivermectin used for the killing of mites like demodectic (red) mange, sarcoptic mange and, in some cases, ear mites.

Heartgard, Iverhart (and the like) used in labeled doses for the prevention of heartworms and other parasites? Not a problem. No reactions noted.

Yet my inbox told another story: Scary, life-threatening reactions the likes of which threaten the lifeblood of a product I currently find incredibly useful…when used with common sense, intelligence and, above all, prudence.

After all, what kind of vet layers parasiticides with nary a thought to their interaction?

Of course, I appreciate the news. I think it’s important to point out the obvious when the obvious may be elusive to some. But to question the safety of an apparently safe product (Comfortis) when used with unapproved doses of a product (ivermectin) whose safety margin is de-facto in question, is NOT the spin on this breaking news Comfortis deserves.

(And, NO, I DO NOT receive a lifetime supply of Lilly’s Comfortis for saying so.)