Zyrtec (cetirizine) is an antihistamine approved for use in humans to treat allergy symptoms. In veterinary medicine it’s used in both cats and dogs for the same indication...and more.

For dogs, I’ll turn to Zyrtec when Benadryl (diphenhydramine) fails. Usually, these are the itchy dogs: the hot spot-ridden, flea allergic, food allergic and/or atopic (inhalant allergic). Except in older dogs whose kidney function I carefully screen before embarking on a course, Zyrtec has proved incredibly safe and moderately effective. The ability to buy it OTC and dose it only once daily for some dogs––not to mention its less drowse-inducing action––has enlisted my fandom.

The only drawback? Its brand name version is more expensive, meaning it tends to be pricier than drugs like Benadryl. And for a drug that sometimes has to be administered for weeks on end, that’s no small factor. Luckily, it's off patent now and you can purchase generics for significantly less than the prettily packaged stuff.

Moderate dog success notwithstanding, where Zyrtec really shines is in my kitty patients. Though it doesn’t work for all itchy cats, it does seem to help quite a bit––far more than Benadryl’s diphenhydramine and significantly more than chlorpheniramine (my former go-to antihistamine for cats).

The dermatologists on VIN (the Veterinary Information Network) seem to agree: Good, safe stuff for cats, this Zyrtec. Probably more effective than the alternatives. And definitely easier because, for cats, we now know that once-a day dosing is perfectly appropriate.

The best news for felines, however, is not just that Zyrtec seems to help for their itchiness, but also that it may help treat eosinophilic diseases.

What’s that, you ask? They’re a collection of [typically] skin, airway and intestinal diseases cats suffer much more frequently than dogs. They can cause stomatitis (oral inflammation), rodent ulcers (unsightly upper lip lesions), eosinophilic plaques (crusty sores), intestinal ulceration and diarrhea, bronchitis and tracheitis and asthma, among other problems.

Lately, it’s been determined that a significant percentage of cats affected by these eosinophilic diseases respond well to Zyrtec. Complete remission of symptoms is actually possible for some once this drug is initiated. So far, this seems to be true for all eosinophilic cases save those of the respiratory variety. (Who knows why.)

A recent case demonstrates the possibilities: A cat remanded to lifelong use of prednisone for her eosinophilic skin disease (manifested primarily in her ears and intestines) was weaned off this harsh, immunosuppressive steroid while Zyrtec was initiated.

I expressed tremendous concern that all the symptoms would almost certainly return, though perhaps at a more manageable level than before the steroid usage. Yet six months later there’s no sign of a break in her remission. No diarrhea. No ear lesions. Nothing. The cat is more playful and happy than she’s ever been.

Though this case is undoubtedly not the norm, the shocking success of it speaks to the need to study this drug in more detail. Currently, most of the evidence in favor of its use comes from the dermatologic community. Too bad the now-vast supply of it is primarily anecdotal.

Luckily, the human medical community has been active in amassing literature on Zyrtec and eosinophilic diseases, leading the veterinary community to begin more aggressively employing it in the hopes that one of the most frustrating feline disease complexes we see in cats can be successfully addressed.

Image: Chez Andre 1 / via Flickr