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 Side Effects. It is possible for a dog being treated with aspirin to experience an upset stomach or ulcers due to being given aspirin. Prolonged aspirin use or overdose can lead to aspirin toxicity, which is characterized by kidney failure, depression or even bloody vomit, among other symptoms.