Solensia is FDA-approved and generally safe for cats with osteoarthritis pain. In the largest clinical trial, 275 cats received either Solensia or a placebo, and the most common side effects (vomiting and diarrhea) occurred at nearly the same rate in both groups: about 25% in treated cats versus 20% in cats that received no active drug. That said, post-market reports have flagged some less common reactions worth knowing about, and certain cats should not receive the drug at all.
How Solensia Works
Solensia contains a lab-made antibody designed specifically for cats. It works by binding to a protein called nerve growth factor (NGF), which plays a central role in sensitizing pain receptors in conditions like osteoarthritis. By capturing NGF before it can activate those receptors, Solensia reduces pain signaling without relying on traditional painkillers like anti-inflammatories. It’s given as a monthly injection under the skin at your vet’s office.
What Clinical Trials Found
In the pivotal trial, digestive issues were the main side effects. Vomiting actually occurred more often in cats receiving the placebo (14.6%) than in treated cats (7%). Diarrhea showed up in about 2 to 7% of treated cats compared to 0% in the placebo group, but the numbers were small. No injection site reactions were observed in any cat during the study.
One reassuring finding involved the immune system. Because Solensia is a protein-based drug, there’s always a concern the body might recognize it as foreign and mount a response against it. Across two field studies covering 259 treated cats, only 4 (1.5%) developed detectable antibodies against the drug. None of those four cats experienced adverse effects from the immune response, and three of the four continued responding well to treatment. The rate was comparable to the placebo group, where 2.3% of cats also showed similar antibody signals.
What Post-Market Reports Show
Since Solensia launched in early 2022, the FDA has received over 5,500 adverse event reports through March 2025. These reports come from pet owners and veterinarians who suspect a connection to the drug, but the FDA is clear that a report does not prove the drug caused the problem. Many of these cats were elderly with pre-existing conditions, making it hard to separate drug effects from disease progression.
The most commonly reported issues include skin reactions (scabs, itching, hair loss, and dermatitis), digestive problems (vomiting, diarrhea, appetite loss), and lethargy. Skin-related reports were notably more prominent in post-market data than in the original clinical trials. Among the more serious reports: 299 cases of euthanasia, 183 deaths, 109 seizures, 107 cases of anemia, 42 reports of kidney failure, and 34 cases of anaphylaxis (a severe allergic reaction). Nine injection-site sarcomas, a type of tumor, were also reported.
Context matters here. With millions of doses administered over three years, these numbers represent a small fraction of treated cats. But the gap between the clean clinical trial data and the messier real-world reports is something to be aware of, particularly the skin reactions and the more serious entries like seizures and anaphylaxis, which didn’t appear in trials.
Which Cats Should Not Get Solensia
Solensia has two firm contraindications. Cats with a known allergy to the drug should never receive it again. And it should not be used in breeding cats, pregnant cats, or nursing queens, because the antibody can cross the placenta and pass into milk.
Cats with advanced kidney disease also deserve special consideration. The European Medicines Agency notes that Solensia has not been studied in cats with stage 3 or stage 4 kidney disease (moderate to severe). Since chronic kidney disease is extremely common in older cats, the same population that develops arthritis, your vet will need to weigh the potential benefits against the unknowns for cats with significant kidney problems.
Skin Reactions to Watch For
The post-market data highlights skin problems more than clinical trials predicted. Scabbing was the single most reported adverse event (621 reports), followed by itching (485 reports) and various forms of dermatitis and skin lesions. Some cats develop facial itching and scratching specifically. These reactions may reflect an immune-mediated sensitivity in a subset of cats. If your cat develops new skin lesions, scabs, or excessive scratching after an injection, that’s worth reporting to your vet promptly. Because Solensia stays in the body for about a month, any reaction needs to be managed until the drug clears naturally.
Long-Term Use
Solensia is designed as an ongoing monthly treatment, not a short course. The clinical trial ran for three months, so the longest controlled safety data covers that window. Many cats in practice have been receiving it for much longer. The very low immunogenicity rate (1.5%) is encouraging for long-term use, since it means most cats’ immune systems continue to tolerate the drug over repeated doses without building resistance or reacting to it.
There are no published long-term studies beyond six months with controlled comparisons, so the long-term safety picture relies heavily on post-market surveillance. If your cat is doing well on Solensia, most vets are comfortable continuing it indefinitely, but periodic check-ins and bloodwork help catch any emerging issues with kidney function or blood counts.
Combining Solensia With Other Pain Medications
Because Solensia works through a completely different mechanism than traditional pain drugs, it doesn’t interact with them the way two anti-inflammatory medications might. It targets a growth factor in the nervous system rather than blocking inflammation directly. In the clinical trials, no specific drug interactions were flagged. Many vets use Solensia alongside other pain management strategies, though the formal interaction data is limited. If your cat is already on other medications, your vet can assess whether combining them makes sense for your cat’s specific situation.