Less serious side effects include: 1  Loss of appetite. 2  Weight loss. 3  Nausea. 4  Vomiting. 5  Diarrhea. 6  Frequent urination. 7  Muscle cramps, painful and swollen joints. 8  Excessive tiredness. 9  Difficulty falling asleep. 10  Headache, nervousness, dizziness. 11  Depression, mood swings. 12  Vivid dreams.
Nausea can occur in dogs for many reasons, particularly when the digestive tract and nervous systems are out of balance. Stress can certainly play a role in nausea.
Aspirin can cause gastrointestinal upsets and ulcers in dogs, just as in humans. You should pay attention to your dog's eating habits when you have been giving it aspirin to watch for signs of an upset stomach. If there are any signs of ulcers, such as blood-tinged vomiting, it is important to stop the aspirin.