If one side of a dog’s mouth suddenly starts drooping, it may be a sign of a problem affecting the facial muscles, nerves, jaw, or surrounding tissues.
What might be noticed is that one side of the lip hangs lower than the other, the mouth looks uneven, or saliva escapes from the affected corner.
Some dogs continue eating, walking, and playing normally despite this change. However, a one-sided facial droop can indicate facial nerve paralysis or another neurological condition that needs veterinary attention.
Table of Contents
Dog's Mouth Drooping on One Side: Why It Happens
A dog’s mouth may droop on one side when the facial nerve becomes damaged or stops controlling the muscles responsible for normal lip and facial movement. This can cause one side of the lip to hang loosely, allowing saliva and food to escape.
Facial nerve dysfunction may result from middle or inner ear infections, trauma, hypothyroidism, tumors, or idiopathic facial paralysis, where no specific cause is identified.
Dental disease, jaw injuries, and other neurological conditions can also produce an uneven appearance around the mouth.
The most important distinction is whether your dog has difficulty moving the facial muscles or whether the mouth is physically unable to close. These can involve different nerves and require different investigations.
A sudden change in facial symmetry should be assessed by a veterinarian immediately, even if your dog otherwise seems healthy.
Dog’s Mouth Drooping on One Side: Causes
1. Facial Nerve Paralysis
Facial nerve paralysis is one of the most important causes of a one-sided drooping mouth in dogs.
It occurs when the facial nerve, which controls the muscles responsible for facial expression, becomes damaged or stops functioning properly.
When one side is affected, the lip may hang loosely, the ear may droop, and saliva may escape from the corner of the mouth. Food may also collect between the lip and teeth or fall out when your dog eats.
Some dogs cannot blink or close the eyelid on the affected side. This is particularly important because reduced blinking can leave the eye exposed to dryness and damage.
Possible causes include ear infections, trauma, hypothyroidism, tumors, and idiopathic facial paralysis, which occurs when no underlying cause can be identified.
2. Middle or Inner Ear Infection
An infection affecting the middle or inner ear can damage or inflame the facial nerve because it passes close to these structures.
If the nerve becomes affected, a dog may develop a drooping lip on the same side as the ear problem.
The dog may also drool, have difficulty blinking, or appear to have an uneven facial expression.
Other signs can include head shaking, scratching one ear, an unpleasant ear smell, head tilting, or loss of balance.
Some dogs develop facial changes without obvious signs of ear discomfort. A veterinary examination of the ears is therefore important when a new facial droop appears.
3. Idiopathic Facial Paralysis
Idiopathic facial paralysis occurs when the facial nerve stops functioning normally without an identifiable underlying cause. It can affect one side of the face and may develop suddenly.
A dog may have a drooping upper or lower lip, reduced ear movement, excessive drooling, and difficulty blinking on the affected side.
Some dogs continue eating and behaving normally, although food and saliva may escape from the drooping side of the mouth.
The diagnosis is made after the veterinarian investigates and rules out other possible causes, such as ear disease, trauma, and certain neurological conditions. Recovery varies, and some dogs have lasting changes in facial movement.
4. Facial Injury or Trauma
An injury involving the head, face, or area around the ear can damage the facial nerve or surrounding muscles.
Possible causes include accidental impacts, falls, bites, and other injuries. Depending on the location, the injury may affect one side of the mouth and cause the lip to hang lower.
Swelling, bruising, tenderness, difficulty chewing, or changes in ear and eyelid movement may be noticed.
If the drooping started after an accident or injury, seek immediate veterinary assessment rather than waiting to see whether the facial appearance returns to normal.
5. Dental Disease or Tooth Abscess
Dental disease can cause pain, swelling, and changes in how a dog holds its mouth. An infected tooth or abscess may make one side of the face appear swollen or uneven.
Bad breath, swollen gums, difficulty chewing, drooling, or reluctance to eat hard food may be noticed.
However, dental pain does not usually cause true facial nerve paralysis. If one lip hangs loosely and your dog cannot move that side normally, a nerve problem should also be investigated.
Your veterinarian may recommend a detailed oral examination and dental X-rays to identify hidden tooth-root problems.
Read more: Tooth Abscess in Dogs (Signs, causes and treatment)
6. Trigeminal Neuropathy (Dropped Jaw Syndrome)
The trigeminal nerve controls the muscles responsible for closing the jaw. When this nerve becomes inflamed or stops functioning normally, a dog may suddenly be unable to close its mouth.
This condition is known as idiopathic trigeminal neuropathy or dropped jaw syndrome.
The mouth hangs open, saliva drips continuously, and a dog struggles to eat or drink.
Unlike facial nerve paralysis, the main problem is weakness of the jaw-closing muscles rather than simply a drooping lip.
Most cases improve over several weeks with veterinary supportive care, but sudden inability to close the mouth requires prompt examination because jaw fractures and other neurological problems can look similar.
7. Jaw Injury or Temporomandibular Joint Problems
The temporomandibular joints connect the lower jaw to the skull and allow your dog to open and close its mouth.
An injury, fracture, or joint disorder can interfere with normal jaw movement and make the mouth appear uneven.
There may be difficulty in chewing, pain when opening the mouth, drooling, or an unusual jaw position. Some dogs also develop swelling around the cheek or lower jaw.
If your dog has recently experienced an accident or suddenly struggles to open or close its mouth, arrange urgent veterinary assessment.
8. Growths Affecting the Mouth or Facial Nerves
Growths around the mouth, jaw, middle ear, or tissues surrounding the facial nerve can sometimes interfere with normal facial movement.
Depending on their location, they may cause swelling, an uneven appearance, drooling, difficulty chewing, or progressive changes in lip movement.
Some growths develop gradually, while others may become noticeable when they begin affecting nearby structures.
A veterinarian may recommend imaging or tissue sampling if an abnormal growth is suspected.
Dog Mouth Drooping on One Side but Acting Normal
A dog may have one side of its mouth drooping while continuing to eat, play, walk, and interact normally.
This can happen with facial nerve paralysis, particularly when the condition affects facial movement without causing obvious discomfort or weakness elsewhere.
However, normal behavior does not mean the problem can be ignored. A dog with facial nerve dysfunction may be unable to blink properly, allowing the affected eye to become dry and irritated.
Watch whether food falls from one side of the mouth, saliva collects at the affected corner, or one ear hangs lower than the other.
Arrange a veterinary examination promptly, even if your dog seems otherwise healthy.
Dog Mouth Drooping on One Side and Drooling
When one side of the mouth droops and saliva escapes from that corner, facial nerve paralysis becomes an important possibility.
The affected muscles may no longer hold the lip in its normal position, allowing saliva to leak out. Food and water may also collect between the lip and teeth.
Dental disease, oral injuries, and jaw problems can also cause drooling, although they may not produce true facial weakness.
If your dog is drooling heavily, struggling to swallow, or unable to close its mouth, seek urgent veterinary attention.
Dog Mouth Drooping on One Side and Eye Drooping
When the mouth and eye droop on the same side, facial nerve paralysis is an important condition to investigate.
The facial nerve controls muscles involved in lip movement, ear movement, and eyelid closure. If it is affected, your dog may have a drooping lip, a lower-hanging ear, and difficulty blinking on that side.
The affected eye may become dry because normal blinking and tear production can be reduced. This can lead to irritation and damage to the surface of the eye if it is not protected.
A veterinarian should examine the eye and facial nerves promptly. Do not wait for redness or discharge to develop before seeking advice.
Senior Dog Mouth Drooping on One Side
A senior dog whose mouth suddenly starts drooping on one side may have facial nerve paralysis or paresis, a condition in which the facial nerve becomes damaged or stops controlling the muscles responsible for facial movement.
The affected lip may hang loosely, making the face appear uneven. The dog may also drool from one side of the mouth, struggle to keep food in its mouth, or have food and water fall out while eating or drinking. In some cases, the dog may also be unable to blink normally or move the ear on the affected side.
In older dogs, possible causes include middle or inner ear infections, hypothyroidism, injuries affecting the facial nerve, and tumors involving the ear or nerves.
However, some dogs develop idiopathic facial paralysis, meaning the cause cannot be identified.
Although facial nerve paralysis is more common in older dogs, a drooping mouth should not automatically be attributed to aging.
A veterinarian should examine the dog promptly to determine the cause.
Seek emergency care if the drooping is accompanied by sudden loss of balance, head tilt, difficulty walking, seizures, collapse, or difficulty swallowing.
Dog’s Mouth Drooping on One Side: What to Do
1. Arrange a Veterinary Examination
Contact your veterinarian promptly, particularly if the change appeared suddenly.
Explain when you first noticed the drooping and whether your dog can move its lips, blink both eyes, move its ears, and close its mouth normally.
A short video of your dog’s facial movements may also help the veterinarian assess the problem.
2. Observe Both Sides of the Face
Compare the two sides of your dog’s face. Look for differences in lip position, ear movement, blinking, and the amount of saliva coming from each side.
Also notice whether food falls out of the affected side when your dog eats.
Avoid repeatedly pulling or manipulating the drooping lip.
3. Check for Possible Ear Problems
Look for head shaking, ear scratching, discharge, or an unpleasant smell.
An infection involving the middle or inner ear can affect the facial nerve, so mention any ear symptoms to your veterinarian.
Do not insert cotton swabs or apply ear medication without veterinary advice.
4. Protect the Affected Eye
If your dog cannot blink or close the eye on the affected side, the eye may become dry and vulnerable to injury.
Contact your veterinarian about appropriate eye lubrication and protection. Do not use human eye drops without professional advice.
5. Monitor Eating and Drinking
Watch whether your dog can pick up food, chew, close its mouth, and swallow normally.
If food or water falls out of one side, tell your veterinarian. If swallowing is difficult, do not force food or water into your dog’s mouth.
6. Avoid Unprescribed Medication
Do not give human painkillers, leftover antibiotics, or other medication without veterinary instructions.
The appropriate treatment depends on whether the problem involves the facial nerve, ear, teeth, jaw, or another structure.
When to Call or Visit Your Vet
A newly drooping mouth on one side should be assessed by a veterinarian immediately, even if your dog is still behaving normally.
Seek emergency veterinary care if you notice:
Sudden inability to close the mouth.
Difficulty swallowing or choking.
Heavy drooling accompanied by weakness.
Inability to blink or close one eye.
Sudden head tilting or loss of balance.
Difficulty walking or weakness in the legs.
Severe facial swelling or suspected injury.
Difficulty breathing.
Collapse or seizures.
In particular, an eye that cannot close properly needs attention because the exposed surface can become damaged.
Dog’s Mouth Drooping on One Side: Treatment
Treatment depends on the underlying cause. Your veterinarian will examine the mouth, ears, jaw, eyes, and facial movements and perform a neurological assessment.
Blood tests, including thyroid testing, may be recommended, while ear examinations, dental X-rays, or advanced imaging such as CT or MRI may be needed to investigate deeper problems.
If facial nerve paralysis is associated with an ear infection, treatment will target the infection.
Dogs with idiopathic facial paralysis may need ongoing eye lubrication and monitoring, while jaw injuries and dental problems require condition-specific treatment.
Dogs with trigeminal neuropathy may need supportive care to maintain adequate food and water intake until nerve function improves.
Key Takeaway
A dog with its mouth drooping on one side may have facial nerve paralysis, an ear infection, a facial injury, dental disease, or a problem affecting the jaw.
Facial nerve paralysis is particularly important when the drooping lip occurs alongside drooling, a hanging ear, or difficulty blinking on the same side.
Even if your dog is eating and behaving normally, a new change in facial symmetry should not be ignored. Arrange a veterinary examination immediately so the cause can be identified and the affected eye and mouth properly protected.
