Dogs get underbites when their lower jaw grows longer than their upper jaw, pushing the bottom teeth out in front of the top teeth. This is extremely common in flat-faced breeds like Bulldogs, Boxers, and Pugs, where it’s actually written into the breed standard as “normal.” But underbites can show up in any breed, and roughly 26% of puppies evaluated in one study of 297 dogs had some form of misaligned bite. Whether an underbite is harmless or a health concern depends entirely on what’s happening inside the mouth.
Genetics Drive Most Underbites
The single biggest reason dogs have underbites is selective breeding. When humans bred dogs for shorter, flatter faces over generations, the upper jaw shortened more than the lower jaw. That mismatch is the underbite. The upper and lower jaws don’t shrink at the same rate because genetic changes to face length affect the upper jaw (maxilla) disproportionately, leaving the lower jaw sticking out beyond it.
Researchers at the University of Edinburgh identified a specific mutation behind this in brachycephalic (flat-faced) breeds. These dogs carry a defect in a gene called SMOC2, which normally guides the development of facial structures including both jaws. In breeds like Pugs, the mutation includes a premature stop signal in the gene’s code, essentially cutting its instructions short. The result is the dramatically compressed face shape, and the underbite that comes with it.
Purebred dogs are significantly more likely to have misaligned bites than mixed breeds. In the puppy study, 33.8% of purebred individuals had a malocclusion compared to 20% of mixed breeds. That gap reflects how concentrated these jaw-shaping genes become through selective breeding within a closed gene pool.
Breeds Where Underbites Are Expected
For several breeds, an underbite isn’t a flaw. It’s the standard. Boxers are one of the most recognizable examples, with their square jaw and prominent underbite considered a defining trait. Bulldogs, Shih Tzus, Boston Terriers, Lhasa Apsos, and Pekingese all share this characteristic. The Pekingese, sometimes called the “lion dog,” typically has both an upturned nose and a noticeable underbite as part of its breed profile.
Veterinary dentists classify these bites as “abnormal” in a technical sense but “normal for the breed.” The distinction matters because it means the underbite itself doesn’t automatically require treatment. What matters is whether the misalignment is causing pain or damage inside the mouth.
How Jaw Growth Creates Problems
A dog’s upper and lower jaws grow at different rates during puppyhood, and the timing of tooth eruption plays a critical role in where the jaw ends up. If the jaws are in an abnormal position relative to each other when the permanent teeth come in and lock into place, the bite gets frozen in that misaligned state. The adult teeth essentially act like a cage, preventing the jaws from correcting themselves with further growth.
Retained baby teeth can make this worse. When a puppy’s deciduous (baby) tooth doesn’t fall out on schedule, it crowds the incoming permanent tooth and can push it into the wrong position. Normally, the pressure of the adult tooth emerging against the baby tooth’s root triggers the baby tooth to loosen and fall out. But if the permanent tooth comes in slightly off-course, the baby tooth stays put, and the crowding locks everything into a worse alignment than the dog might have ended up with otherwise.
When an Underbite Causes Real Problems
Many dogs live perfectly comfortable lives with underbites and never need any intervention. The line between cosmetic quirk and medical issue comes down to one question: are teeth hitting things they shouldn’t? If a tooth contacts soft tissue or another tooth in a way that causes wear, erosion, or trauma, that’s a problem vets call a “traumatic malocclusion,” and it needs attention.
One of the most common complications is a lower canine tooth that sits so upright it digs into the hard palate (the roof of the mouth). Over time, this can wear a groove or even a hole through the tissue, creating what’s called an oronasal fistula, an abnormal opening between the mouth and the nasal cavity. That’s not just painful. It can lead to chronic nasal infections and significant discomfort.
Crowded teeth are another concern. When misalignment pushes teeth too close together, plaque builds up in spots that are nearly impossible to clean, leading to gingivitis and gum disease. Signs that an underbite is causing your dog trouble include unusually bad breath, bloody drool, difficulty picking up food, rubbing the face against walls or with paws, and flinching when touched on the head or muzzle.
Treatment Options for Painful Underbites
When a vet determines that an underbite is causing trauma inside the mouth, three main approaches exist: extraction, crown reduction, and tooth movement.
- Extraction is the most common choice and provides immediate relief. If a lower canine is gouging the palate, removing it solves the problem, though these teeth have very long roots and the procedure can be challenging.
- Crown reduction shortens the offending tooth so it no longer reaches the tissue it was damaging. The tooth stays in the mouth but needs follow-up work to protect the exposed inner layer, either through a medicated dressing that allows healing or a full root canal followed by a protective composite cap.
- Tooth movement is essentially orthodontics for dogs. Vets use inclined planes (appliances bonded to the teeth), small brackets similar to human braces, or elastics to gradually shift teeth into a position that eliminates the trauma.
None of these treatments aim to give the dog a “normal” bite for cosmetic reasons. The goal is always to eliminate pain and prevent tissue damage.
Caring for a Dog With an Underbite
Because misaligned teeth create more nooks where plaque accumulates, daily brushing is especially important for dogs with underbites. Start by getting your dog used to having a gauze-wrapped finger rubbed along the teeth and gums for about a week before introducing a soft-bristled toothbrush with pet-safe toothpaste. Hold the brush at a 45-degree angle so the bristles reach just under the gumline, and use a circular motion. You only need to focus on the outer surfaces of the teeth at first.
Making brushing a consistent daily habit at the same time each day helps your dog accept it as routine rather than something to resist. Between brushings, chewing activity helps keep plaque in check. When choosing chew toys, use the fingernail test: if you can press your fingernail into the material and leave a dent, it’s soft enough to be safe. Anything harder risks cracking teeth, which is the last thing a dog with an already tricky bite needs.
Regular veterinary dental checkups let you catch early signs of tissue damage or gum disease before they become serious. For many dogs with mild underbites, consistent home care and routine monitoring are all that’s ever needed.