The answer to your question really depends on what kind of toothpaste you’re using. Human toothpaste isn’t recommended for dogs because they will likely swallow the paste, which contains fluoride that can cause gastrointestinal irritation or more significant problems in large amounts.
Toothpaste formulated for pets is generally safe. Minor stomach upset could occur if your pet swallows more than what is normally used for brushing. Pet dental products that contain an ingredient known as xylitol should be used carefully and only according to label directions. When ingested in amounts that exceed the recommended per label dose, xylitol can cause a sudden drop in a dog's blood sugar, resulting in weakness, tremors and seizures as well as the potential for liver failure. Should an accidental over ingestion occur, contact your local veterinarian or the ASPCA Animal Poison Control Center at 888-426-4435 for immediate assistance.