Have a headache or muscle pain? Many of us will go grab the bottle of Aleve, but few know that if we gave that pill to our pets or accidentally dropped it, it could prove fatal. Naproxen belongs to the class of nonsteroidal antiinflammatory medications (NSAIDs) along with aspirin, acetaminophen, and ibuprofen. It is also commonly found in combination with cough and cold medications, joint medications, and even prescription pain medications. Naproxen can cause serious symptoms in dogs and cats, with cats being especially sensitive to its effects.