Dogs should never take Flexeril, according to the Merck Veterinary Manual. The veterinary manual states that Flexeril, also known as cyclobenzaprine, is toxic to canines and can cause severe reactions.
The Merck Veterinary Manual states that symptoms of Flexeril poisoning in dogs include vomiting, weakness, seizures and hypothermia. Merck explains that dog owners should induce vomiting in the event that their dog consumes, or is suspected to have consumed, the drug. Petmd notes that vomiting can be induced by giving a dog a dose of hydrogen peroxide; the proper dosage of hydrogen peroxide to induce vomiting is 1 teaspoon for every 5 pounds of body weight. Vomiting should only be induced within two hours of the time the dog ingests the drug, according to PetMD.