VetMedGuide

Vet Med Guide

Pharmaceuticals2026-05-14 · 12 min read

Zenrelia for Dogs: Vaccine Timing, Boxed Warning, and Patient Selection

Zenrelia (ilunocitinib) is a once-daily oral JAK inhibitor for canine allergic dermatitis.

Ran Chen
Ran Chen
Founder, VetMedGuide. Life-sciences operator and 10× global market-access lead.
Published

Zenrelia (ilunocitinib tablets) is a once-daily oral Janus kinase (JAK) inhibitor approved by the FDA in September 2024 for control of pruritus associated with allergic dermatitis and control of atopic dermatitis in dogs at least 12 months of age. Manufactured by Elanco, it is the newest entrant in the JAK inhibitor class that already includes oclacitinib (Apoquel).

Zenrelia launched with a prominent boxed warning about vaccine safety that generated significant concern among veterinarians and pet owners. In September 2025 — one year after approval — the FDA approved a label change that removed the most alarming language. Understanding what the boxed warning says now, what was removed, and what remains is essential for any veterinarian considering this drug or any pet owner whose dog has been prescribed it.

This article covers the label as it stands in 2026, including the updated warning, vaccine timing requirements, clinical trial data, adverse events, and patient selection.

How Zenrelia works

Ilunocitinib is a non-selective JAK inhibitor. It primarily acts on JAK1, JAK2, and TYK2 enzymes, inhibiting the function of pruritogenic, pro-inflammatory, and allergy-related cytokines that depend on JAK signaling. It is not a corticosteroid or an antihistamine. By blocking these cytokine pathways, Zenrelia reduces both itch (pruritus) and skin inflammation (dermatitis).

Like oclacitinib, Zenrelia works rapidly — pruritus reduction is typically observed within the first day to week of treatment.

Dosing and administration

Zenrelia is dosed at 0.6 to 0.8 mg/kg (0.27 to 0.36 mg/lb) orally, once daily, with or without food. There is no loading dose or taper requirement. The drug can be started and stopped as needed for short-term or long-term use.

Tablets are available in four scored strengths: 4.8 mg, 6.4 mg, 8.5 mg, and 15 mg. The scoring allows tablets to be split for precise dosing. Tablet selection is based on body weight, following the dosing table in the prescribing information.

The boxed warning: what it says now

The current boxed warning on the U.S. Zenrelia label reads:

WARNING: INADEQUATE IMMUNE RESPONSE TO VACCINES. Based on results of the vaccine response study, dogs receiving Zenrelia are at risk of an inadequate immune response to vaccines. Discontinue Zenrelia for at least 28 days to 3 months prior to vaccination and withhold Zenrelia for at least 28 days after vaccination. Dogs should be up to date on vaccinations prior to starting Zenrelia.

Additional warnings in the label include:

  • Do not use in dogs less than 12 months old
  • Do not use in dogs with a serious infection
  • Monitor for infections — Zenrelia may increase susceptibility to infections including adenoviral hepatitis and pancreatitis, demodicosis, interdigital furunculosis, coccidiosis, and pneumonia
  • Neoplastic conditions (benign and malignant) were observed during clinical studies
  • Hematologic effects — Zenrelia may cause progressive or persistent decreases in hematocrit, hemoglobin, and/or red blood cell count without a corresponding increase in reticulocytes

The safe use of Zenrelia has not been evaluated in breeding, pregnant, or lactating dogs. Concurrent use with glucocorticoids, cyclosporine, or other systemic immunosuppressive agents has not been tested.

What the label change removed — and why

At launch, the boxed warning was more alarming. It included language about "fatal vaccine-induced disease from modified live virus vaccines," based on the outcome of a laboratory study where 8 young, unvaccinated dogs were dosed at three times the label dose of Zenrelia and then vaccinated. One dog in that study developed fatal adenoviral hepatitis and pancreatitis, and a second dog died from infectious enteritis that potentially contributed to a fatal intussusception.

In September 2025, the FDA approved removal of the "fatal vaccine-induced disease" language from the U.S. label. The reason: additional PCR data from the original study showed that canine adenovirus type 1 (CAV-1, the agent of infectious canine hepatitis) was present in liver samples from the affected dogs, consistent with a natural outbreak of infectious canine hepatitis rather than vaccine-induced disease. Multiple dogs in the study also had concurrent Cystoisospora canis infections, suggesting underlying health compromise in the study population.

The FDA concluded that "the totality of evidence supports removal of the risk of fatal vaccine-induced disease from modified live virus vaccines from the labeling."

The boxed warning about inadequate immune response to vaccines remains. This means that while fatal vaccine-induced disease is no longer listed as a risk, the drug's immunosuppressive properties still warrant caution around vaccination timing.

Notably, Zenrelia's label in other countries — including EU member states, the United Kingdom, Brazil, and Japan — never included the fatal vaccine-induced disease warning. Some countries defer vaccine timing decisions to veterinarian discretion rather than mandating specific washout periods.

Vaccine timing in practice

The label requirements for vaccine timing are:

  1. Before starting Zenrelia: Ensure the dog is up to date on all vaccinations.
  2. Before vaccination: Discontinue Zenrelia for at least 28 days to 3 months.
  3. After vaccination: Wait at least 28 days before resuming Zenrelia.

The 28-day to 3-month pre-vaccination washout is based on data showing recovery from drug-induced immunosuppression occurred 27 to 83 days after stopping Zenrelia in the vaccine response study. A 3-month washout for immunosuppressants before vaccination is consistent with 2024 WSAVA vaccination guidelines and 2023 CDC vaccination guidelines for immunocompromised humans.

The 28-day post-vaccination hold is based on data about the duration of viral shedding after modified live virus vaccine administration.

In practice, this timing creates a gap of roughly 2 to 4 months during which the dog cannot receive Zenrelia around a vaccination event. For a dog with severe pruritus, that gap may be clinically significant. Options during the gap include:

  • Cytopoint (lokivetmab): A monoclonal antibody against IL-31 that does not interfere with vaccine response and has no washout requirement. It can bridge the gap.
  • Topical therapy: Antimicrobial shampoos, leave-on conditioners, and topical corticosteroids for localized flares.
  • Short-course glucocorticoids: For acute flares, with awareness of side effects and the fact that concurrent use with Zenrelia has not been evaluated.

Clinical trial results

Zenrelia's approval was supported by two pivotal placebo-controlled field trials and one head-to-head trial against oclacitinib.

U.S. atopic dermatitis trial (vs. placebo)

In a multicenter, double-masked, randomized, placebo-controlled trial of 268 client-owned dogs with atopic dermatitis, ilunocitinib (0.6 to 0.8 mg/kg once daily) was compared to placebo for 112 days:

  • Day 28 treatment success (≥50% reduction in pruritus Visual Analog Scale or CADESI-04): 83% of ilunocitinib-treated dogs vs. 31% of placebo dogs (p < 0.001)
  • Clinical remission of pruritus (PVAS < 2): Significantly more ilunocitinib-treated dogs from Day 7 onward
  • Clinical remission of skin lesions (CADESI-04 < 10): Significantly more ilunocitinib-treated dogs from Day 14 onward
  • Two-thirds of dogs achieved clinical remission after 4 months of treatment

U.S. allergic dermatitis trial (vs. placebo)

In a trial of 306 dogs with allergic dermatitis, treatment success at Day 7 was 25.4% for ilunocitinib vs. 7.7% for placebo. By Day 28, significantly more ilunocitinib-treated dogs achieved clinical remission.

Head-to-head trial (vs. oclacitinib)

In a European multicenter, double-blind study of 338 client-owned dogs with confirmed atopic dermatitis, once-daily Zenrelia showed similar efficacy to oclacitinib (dosed twice daily for 14 days, then once daily) at the primary endpoint on Day 28. Adverse event rates were similar between groups.

A separate head-to-head study published in Veterinary Dermatology in 2025 by Forster et al. found that once-daily ilunocitinib offered significantly better long-term control of pruritus and skin lesions from Day 28 to Day 112 compared to oclacitinib, with more dogs achieving clinical remission of pruritus.

Adverse reactions

Adverse reactions through Day 112 in the U.S. placebo-controlled trials (N = 181 Zenrelia, N = 87 placebo):

Adverse reaction Zenrelia Placebo
Vomiting or nausea 22.1% 16.1%
Diarrhea 19.9% 10.3%
Lethargy 12.2% 10.3%
Otitis externa 10.5% 23.0%
Anorexia 9.4% 8.0%
Dermal growth (cyst, papilloma) 8.8% 4.6%
Epiphora or ocular discharge 7.7% 1.1%
Coughing or wheezing 6.6% 2.3%
Elevated liver enzymes 5.5% 2.3%
Urinary tract infection 5.5% 2.3%
Leukopenia 4.9% 1.1%

The gastrointestinal effects (vomiting, diarrhea) are the most common and are typically mild and transient. The leukopenia signal and the neoplastic conditions observed (including both benign and malignant growths) are consistent with the immunosuppressive mechanism of JAK inhibition and warrant monitoring.

In a 6-month target animal safety study, dogs dosed at 1, 2, 3, and 5 times the label dose all completed the study with no serious adverse events. Dose-related findings included generalized demodicosis, gum infections, interdigital cysts, papillomas, and swollen feet.

Monitoring

The label does not specify a mandatory monitoring schedule, but based on the safety profile, reasonable monitoring includes:

  • Baseline bloodwork (CBC, chemistry panel) before starting Zenrelia, particularly to establish hematocrit and liver enzyme values
  • Periodic CBC during treatment to detect progressive anemia or leukopenia
  • Regular skin and ear examinations at recheck visits to monitor for secondary infections and new dermal growths
  • Assessment for signs of infection at every visit — demodicosis, interdigital furunculosis, urinary tract infections, and respiratory infections were all observed in clinical trials

Which dogs are candidates — and which are not

Good candidates:

  • Dogs at least 12 months old with confirmed allergic or atopic dermatitis
  • Dogs that are up to date on vaccinations
  • Dogs without active serious infections
  • Dogs where daily oral medication is acceptable to the owner
  • Dogs that have not responded adequately to or cannot tolerate other therapies (cytokine therapy, other JAK inhibitors, immunotherapy)

Not candidates or require special caution:

  • Dogs under 12 months of age (the immune system is still developing, and the drug has not been evaluated in this age group)
  • Dogs with active serious infections
  • Dogs with a history of recurrent infections or demodicosis
  • Dogs that are due for vaccinations in the near future and cannot tolerate the washout period
  • Breeding, pregnant, or lactating dogs (safety not established)
  • Dogs currently on systemic glucocorticoids or cyclosporine (concurrent use not tested)

A 2025 survey published in JAVMA found that veterinarians selected ilunocitinib less often than lokivetmab and oclacitinib regardless of financial constraints, and the authors suspected that "the newness of this medication and the package insert warning were responsible," since head-to-head data demonstrated comparable or superior efficacy.

How Zenrelia compares to Apoquel

Both drugs are non-selective JAK inhibitors. The key differences:

  • JAK binding profile: Zenrelia primarily acts on JAK1, JAK2, and TYK2. Oclacitinib primarily acts on JAK1 and JAK3. In vitro assays do not predict clinical differences — head-to-head trials remain the standard.
  • Dosing frequency: Zenrelia is dosed once daily from the start. Oclacitinib is dosed twice daily for the first 14 days, then once daily.
  • Vaccine warning: Zenrelia carries a boxed warning about inadequate immune response to vaccines and requires a washout period. Oclacitinib does not carry a specific boxed warning about vaccines, though as an immunosuppressant it also warrants caution.
  • Efficacy: The head-to-head trial showed similar efficacy at Day 28, with Zenrelia showing advantages in long-term control at later time points.
  • Cost: Pricing varies by pharmacy and region; Zenrelia is priced competitively with oclacitinib in most markets.

The existing VetMedGuide article on Zenrelia vs. Apoquel covers the comparison in more detail.

What owners should know

If your veterinarian has prescribed Zenrelia:

  • Your dog should be current on all vaccines before starting the drug. Check with your veterinarian if any vaccinations are due.
  • Tell your veterinarian about all medications and supplements your dog is taking, especially steroids, cyclosporine, or other immune-modulating drugs.
  • Watch for GI upset in the first few weeks — vomiting and diarrhea are common but usually mild. Contact your veterinarian if they persist.
  • Monitor for signs of infection: lethargy, coughing, skin changes, or limping. Zenrelia suppresses parts of the immune system, and infections can emerge or worsen.
  • Do not stop or restart the medication without veterinary guidance, especially around vaccination appointments.
  • Bloodwork at baseline and periodically helps detect the hematologic changes (anemia, low white blood cell count) that can develop during treatment.

Sources

Topics
Dermatology
Related Articles