More than 12.2mg per 2.2-pound weight of Flexeril can be toxic for the dog. If you give your dog 1.1 mg per 2.2 pounds of body weight, it will not have any toxic effect. However, your dog can have some side effects like vomiting, drooling or heartbeat becoming high. The truly toxic dosage of Flexeril is 12.2 mg per 2.2 pounds of your dog's body weight.