As you read all of these adverse reactions, remember that only a few dogs will have these reactions, and that they have not yet been definitively linked to Rimadyl usage. There may have been other causes, especially since the average Rimadyl user tends to be an older dog. The most serious side effect seen is a rare liver reaction. When a drug reaction is seen, it's because the drug is metabolized into something that is toxic to that individual's liver. It causes death of some of the liver cells.
Liver problems show up first as a loss of appetite, vomiting, or diarrhea.
Of the 40 or so dogs with liver problems, about half were Labrador Retrievers. It's not clear whether this indicates a problem in the breed's metabolism of Rimadyl or if it's because there are so many Labs on the drug. These reactions, while rare, are serious and potentially life-threatening . The sooner the condition is recognized, the more likely the chances of recovery. Late last summer, Pfizer sent a letter to all veterinarians alerting them to the liver complications.