Also know, can I take cephalexin that was prescribed to my dog? People and other animals can use it, too. Veterinarians usually prescribe cephalexin to dogs as an oral medication that can come in liquid, tablet, or capsule forms. If your veterinarian has prescribed cephalexin for your dog, then you should ask about proper dosage and possible side effects. How long does it take antibiotics to work for kennel cough?