There are many reasons why a dog's mouth may be swollen, depending on the dog's health and environment. Here are some common causes:
Dental disease: Your dog’s mouth may be swollen because of dental problems, such as gingivitis or retained teeth.
Allergies: Dogs may be allergic to food, medicine, or other things, causing their mouth to swell.
Infection: Dogs may be infected with bacteria, viruses or fungi, causing their mouths to swell.
Others: A dog’s mouth may be swollen due to trauma, tumors or other health problems.
If your dog's mouth is swollen due to any of these reasons, he may need treatment. It is recommended that you take your dog to the veterinarian immediately to determine the cause and receive appropriate treatment.