Monoclonal antibody drugs are becoming more common in veterinary medicine. You may already know some of them. Librela is used to control osteoarthritis pain in dogs, while Solensia is used for osteoarthritis pain in cats.
In September 2026, the U.S. Food and Drug Administration (FDA) released new draft guidance about how veterinary monoclonal antibody drugs should be studied for safety.
This does not mean that the FDA has declared these drugs unsafe. It also does not change the approval status of drugs already on the market. The document is a proposed framework for how companies and regulators should evaluate the safety of this growing group of medications.
But there are several important points for pet owners.
Monoclonal antibodies are different from traditional drugs
A monoclonal antibody is a specially designed protein that attaches to a particular target in the body.
That can make these medications very specific. However, the target of a drug may have more than one job in the body.
Because of this, the FDA says safety testing should look beyond whether a medication produces its desired effect. Researchers should understand where the drug’s target is found throughout the body and what could happen when that target is blocked or changed.
In other words, a drug may do exactly what it was designed to do while still producing an unwanted effect somewhere else.
The FDA wants researchers to look for both “on-target” and “off-target” effects
The new guidance discusses two important types of unwanted effects.
An unintended on-target effect happens when the drug binds to the correct target, but blocking or changing that target causes another biological effect that was not intended.
An off-target effect occurs when the antibody binds to something other than its intended target.
The FDA recommends that both possibilities be considered when developing and testing veterinary monoclonal antibody medications.
This is important because the effects of blocking a biological pathway may extend beyond the reason the drug is being prescribed.
Healthy research animals may not tell us everything
One of the most important parts of the guidance is the FDA’s recognition that laboratory animals and real-world patients can be very different.
Early safety studies commonly involve healthy animals. But the pets receiving these medications in veterinary clinics may be older and may have arthritis, kidney disease, neurologic disease, cancer, or other health problems. They may also be receiving several medications at the same time.
The FDA specifically notes that the way a drug’s target is expressed in the body can differ between healthy laboratory animals and animals with disease.
For that reason, information from healthy laboratory animals alone may not always be enough to understand safety in the animals that will actually receive the medication.
Larger real-world studies matter
Another limitation is numbers.
Laboratory safety studies usually involve relatively small groups of animals. That means an uncommon side effect may never appear during the original study.
The FDA specifically notes that infrequent adverse reactions may not be identified in laboratory studies because of the small number of animals and limited treatment duration.
That is why larger field studies are important.
These studies involve the types of animals that veterinarians actually treat, including animals with other diseases, different genetic backgrounds, and other medications.
Continued safety monitoring after a medication reaches the market also remains important for identifying uncommon problems.
The immune system also needs to be studied
Because monoclonal antibodies are proteins, the animal’s immune system can sometimes recognize them and respond to them.
The FDA recommends evaluating what is called immunogenicity, which means the ability of the drug to cause an immune response.
An immune response could have several outcomes. The medication might become less effective over time. The animal could develop an adverse reaction. In other cases, the immune response might have no noticeable clinical effect.
The FDA also recommends evaluating whether a monoclonal antibody could unintentionally change or suppress normal immune function.
Safety testing should match how the drug will actually be used
The FDA recommends testing the medication using the intended dose, route of administration, treatment interval, and repeated dosing schedule.
That last point matters.
Many monoclonal antibody medications are not given just once. Some pets may receive injections repeatedly for months or years.
Safety studies therefore need to consider what happens with repeated exposure, not simply what happens after one injection.
What does this mean for your dog or cat?
The biggest message is not that monoclonal antibody drugs are “good” or “bad.”
It is that they are powerful biological medications, and their safety needs to be studied differently from many traditional drugs.
The FDA’s new guidance recognizes several important ideas:
- Rare problems may not appear in small laboratory studies.
- Healthy research animals may respond differently from older pets with disease.
- Blocking the correct biological target can sometimes have unintended effects elsewhere in the body.
- Researchers should look for effects on the immune system.
- Repeated treatment needs appropriate safety evaluation.
- Real-world studies are an important part of understanding safety.
For pet owners, this is another reason to keep an open conversation with your veterinarian when your dog or cat receives any long-term medication.
If your pet develops a new medical problem after starting a medication, tell your veterinarian, even if the problem does not seem obviously related to the drug. Your veterinarian can determine whether additional testing is appropriate and whether the event should be reported to the drug manufacturer or the FDA.
One final point
This FDA document is currently draft guidance, not a final regulation. It provides recommendations for how veterinary monoclonal antibody products should be evaluated for safety.
It also does not say that Librela, Solensia, or another specific monoclonal antibody has caused a particular adverse effect.
What it does show is that regulators recognize that monoclonal antibodies require a thoughtful approach to safety. Understanding the biology of the target, studying animals that resemble real patients, looking for uncommon adverse events, and continuing to collect safety information are all important parts of that process.
As more monoclonal antibody treatments enter veterinary medicine, that type of careful safety evaluation will become increasingly important.
Source: U.S. Food and Drug Administration. Target Animal Safety Evaluation for Veterinary Monoclonal Antibody Products, VICH GL62, Guidance for Industry, Draft Guidance. September 2026.
Full FDA document: https://www.fda.gov/media/194837/download
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