Giving a cat medicine every day can be hard. An injection that helps arthritis pain for three months sounds appealing. Fewer pills. Fewer trips to the vet. More comfortable days at home.
A new study published in The Veterinary Journal looked at relfovetmab, sold as Portela. This drug blocks nerve growth factor, or NGF. It belongs to the same family of drugs as Solensia for cats and Librela for dogs. However, it is a different drug, given on a longer dosing schedule.
The study found that relfovetmab helped reduce arthritis pain. That matters. But it does not answer all of my concerns about this drug family.
What did the study find?
Researchers studied 307 pet cats with arthritis at 39 veterinary clinics in Europe. Their median age was 11 years.
About half received relfovetmab. The others received saline, an injection without medicine. Neither the owners nor the vets doing the exams knew which treatment each cat received.
Cats received an injection every three months, for three injections. The study followed them for nine months.
Owners scored how hard it was for their cats to do three everyday activities. At three months, about 73 out of 100 cats receiving the drug met the study’s goal for improvement. About 47 out of 100 cats receiving saline also met that goal.
That is a meaningful difference. Improvement was seen as early as three days after the first injection. Veterinary pain exams also favored the drug.
Still, “treatment success” meant a certain improvement in an activity score. It did not mean that every successful cat was pain-free or that its arthritis had healed.
Feeling better does not prove a joint is healthier
This is one of my main concerns about anti-NGF drugs.
NGF helps carry pain messages. Blocking it can make a pet feel more comfortable. But reducing pain does not, by itself, show that damaged cartilage or bone is getting better.
Think of pain as a warning sound. Turning down that sound can bring relief. However, we still need to check what is happening inside the joint.
The study used X-rays to confirm arthritis before treatment. It did not describe a planned series of repeat X-rays to track changes in joint damage over time.
So, this study supports pain relief. It does not establish that relfovetmab protects joints or rules out faster joint damage.
My concerns include rapidly progressive osteoarthritis, or RPOA, and abnormal changes in bone described as, atypical periarticular bone remodeling, or APBR. In plain language, I am watching for joint damage or unusual bone changes that develop faster than expected. This paper does not prove that relfovetmab causes these problems because it was not checking for them.
Check the joints before treatment, and keep checking
Before starting treatment, I recommend getting radiographs, or X-rays, of the affected joints to help confirm the diagnosis. A cat moving poorly may have arthritis, but weakness or trouble walking can also come from other problems.
Those first images also give us a starting point. If a joint changes later, we have something to compare it with.
The veterinarian should record how the joints feel before treatment. Are matching joints on the left and right sides similar? Is there pain, thickening, swelling, or extra fluid in the joint? That extra fluid is called effusion. Joint movement and muscle size should also be checked.
Pet owners can help establish this baseline. Ask your veterinarian to show you how to gently feel and compare the affected areas at home. Notice differences in size, warmth, swelling, and your cat’s response to light touch. Do not squeeze a sore joint or force it to bend.
Short videos of walking and familiar activities can also help document the starting point.
I recommend repeating these checks at regular intervals during treatment. The veterinarian should repeat the joint exam, and owners should continue gentle checks and keep notes at home. For example: a veterinary exam every 3 months with palpation of the joints. For cat owners: feeling their pet’s joints on a weekly basis. If something doesn’t feel right, get a veterinary exam as soon as possible.
New swelling, increasing joint size, loss of movement, changes in how the cat stands, or worsening lameness may call for repeat X-rays or other tests. These checks can help us look for RPOA or APBR. Feeling the joints alone cannot diagnose either problem, and less pain does not rule out worsening damage.
What about safety?
Health problems were reported in about 44% of cats receiving relfovetmab and 40% receiving saline. These were health events during the study, not all proven side effects.
Pain during injection was more common with the drug: about 7% compared with 1% with saline.
Serious health events occurred in about 9% of treated cats and 7% of saline-treated cats. Seven cats in each group died or were euthanized during the study or soon after leaving it.
The examining vets judged these serious events unrelated to treatment. That finding belongs in any fair discussion of this paper.
But some details deserve attention.
Among the treated cats that died or were euthanized, the reported reasons included spinal cord compression with metastatic cancer, worsening spinal cord compression with weakness, and an inability to walk.
These descriptions do not prove the drug caused those problems. Cancer and spinal disease can cause severe decline on their own. The paper also does not give enough case detail to independently settle every question.
For me, these reports support the need for closer study of new weakness, walking problems, and loss of function during treatment.
The cats selected for a study matter
The trial excluded cats with several conditions, including pain or lameness from a primary nerve disorder, a history of signs of disc disease, and recent injuries causing nerve problems.
Those rules help researchers study arthritis pain more clearly.
They also limit what we can conclude. Results from selected cats with arthritis cannot establish safety for every cat with weakness, poor balance, or spinal disease.
Nine months of follow-ups also cannot answer every question about treatment over several years. With 153 cats receiving the drug, uncommon problems may be missed.
A longer-lasting injection needs careful thought
Three months of relief could be helpful. But once an injection is given, we cannot take it back.
That means we need a plan if a cat develops a problem while the treatment is still having an effect.
Owners should know what changes to watch for and when their cat will be checked again. Fewer injections should not mean fewer chances to assess the patient.
Why I remain cautious
The study was funded by Zoetis, and all authors were Zoetis employees during the research. That does not make the findings wrong. The study’s random assignment, saline comparison, and masked exams are real strengths. Independent research would add confidence.
My concerns about Solensia, Librela, and other anti-NGF drugs remain focused on joint health, nerve function, and safety over time. Drugs in the same family may have different risks. We should examine each one carefully.
New weakness, stumbling, joint swelling, or a sudden loss of mobility deserves a veterinary exam. An inability to stand or walk needs urgent attention.
I want cats to hurt less. I also want to know that their joints and nervous systems remain healthy while we help them feel better.
This study gives us useful evidence of pain relief. It leaves important safety questions open.
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