Amitriptyline may cause numerous side effects in animals including drowsiness, weight gain, loss of appetite, reduced grooming, bladder stone formation and urine retention. However, these instances are rare and mild. An overdose of Amitriptyline can have serious impact on the heart. Pets that take in large amounts of this drug should be quickly taken to a veterinarian. Amitriptyline is a prescription drug and must be obtained only by prescription from a veterinarian.