When Zenrelia first hit the scene in late 2024 into early 2025, I, like most veterinarians, found myself shaking my head. Why would a brand-new dermatologic product be approved, but also come with a black box warning about the risk of fatal vaccine-induced disease, inadequate immune response to vaccines, and a 3 to 4 month washout period in order to vaccinate? That, for me, was a non-starter as I did not feel like the risks outweighed the reward, so I wrote the medication off and decided it wasn't going to be part of my dermatology arsenal. I mean, why risk it when the majority of my patients are well controlled on Cytopoint and Apoquel? And if those didn’t fully do the trick (which wasn’t often), I could always add in Cyclosporine or a little Temaril-P if needed.
But as time went on, I found myself running into the medication more and more. Working with Evette, I get the privilege of working in different veterinary clinics around town and seeing how each hospital approaches medicine differently. I get to see what works, what doesn't, and what our colleagues are comfortable using. What I noticed with Zenrelia was that more and more clinics were slowly starting to incorporate it into their pharmacy. Even if I didn’t personally prescribe it to any of my patients, I knew it was only a matter of time before I would have a patient presenting for routine vaccines while also taking Zenrelia. Not to mention, as time went on, I found more and more patients not being adequately controlled on Apoquel and/or Cytopoint alone.
Thankfully, one of the hospitals where I provide relief services had an associate who shared this article from ‘The Allergic Dog Blog’, written by a board-certified veterinary dermatologist, Dr. Meagan Painter. The article reviews a rigorous 2025 peer-reviewed study that challenged many of the concerns raised by Zenrelia’s original research study that led to the FDA's black box warning.
As I read through the article, one thing became immediately apparent: the original study and the newer study were asking two very different questions. The original study evaluated young, vaccine-naïve Beagles receiving their first vaccinations, with only one test group receiving three times the labeled dose of Zenrelia. To further complicate matters, the research colony experienced outbreaks of infectious diseases during the study, making it difficult to determine how much of the poor vaccine responses were due to the drug itself versus the unfortunate circumstances surrounding the trial. While the FDA was appropriately cautious based on the available data at the time, the study didn't closely resemble the typical allergic adult dog we see in general practice.
The newer 2025 study, however, looked much more like the patients we treat every day. Researchers evaluated previously vaccinated adult dogs receiving routine booster vaccinations while remaining on Zenrelia. They compared dogs receiving the labeled dose, dogs receiving three times the labeled dose, and an untreated control group. The results were reassuring. Dogs receiving Zenrelia developed protective antibody responses to rabies, distemper, parvovirus, and adenovirus that were comparable to those of dogs not receiving the medication. Even the dogs receiving higher than the labeled dose maintained appropriate immune responses.
For me, this was the data I needed to be more comfortable prescribing such medication. One of my biggest concerns with Zenrelia had always been the practical implications and logistics of stopping therapy for 3 to 4 months every time a patient needed routine vaccinations. Many allergic dogs are finally comfortable after months or years of itching, secondary skin infections, and frustrated owners. Asking those patients to discontinue an effective medication for weeks—or even months—around vaccination seemed far from ideal. This newer study suggests that for previously vaccinated adult dogs, that concern may not be nearly as significant as originally feared.
That doesn't mean the black box warning should simply be ignored. The FDA label still recommends a washout period around vaccinations, and veterinarians should continue to exercise clinical judgment for each patient. In addition, all these decisions should ALWAYS be made in the context of a valid Veterinary-Client-Patient-Relationship. It’s also very important to point out that these new findings apply only to previously vaccinated adult dogs greater than 12 months of age, and Zenrelia remains approved only for dogs 12 months of age and older. The newer study did not evaluate vaccine-naïve puppies or dogs in the middle of receiving their puppy vaccine booster series, so we cannot assume the same reassuring results would apply to that type of population. Until more evidence is available, completing the primary vaccine series before starting Zenrelia remains the safest and most evidence-based approach.
Has this article completely erased every concern I have about Zenrelia? No. Like every immunomodulating medication, it still requires thoughtful patient selection and careful monitoring. But it has absolutely changed my perspective. Rather than dismissing the drug because of its original black box warning, I now view it as another valuable option for managing allergic disease in appropriately selected adult canine patients. Having another effective medication available is never a bad thing, especially for those frustrating allergy patients who don't respond well to our traditional therapies, or whose owners are looking for alternatives. Also, supposedly Zenrelia is more effective at controlling not just the itch but the inflammatory component in the patient’s skin, compared to Apoquel and Cytopoint. For me, time will tell on that, but I am excited to start working this medication into my pharmaceutical tool belt for the appropriate patients and gathering that clinical data for myself. As a former practice owner, I often took the same approach regarding bringing new medications into my hospital; waiting until certain new medications had proven themselves in the market and seeing other doctors use them with confidence before putting them on my shelves. With perspective, I’m glad I can now be one of those doctors on the front lines helping to gather such information.
The Allergic Dog - Zenrelia Vaccines
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