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There are several medical causes of aggression in dogs and being aware of them is important. Just like people, dogs can get grumpy when they are feeling under the weather or when feeling pain. This may lower their threshold for aggression and cause them to lash out. Veterinarian Dr. Ivana covers the medical causes of aggression in dogs. 

The Importance of Seeing the Vet

According to its official definition, aggression is a harmful behavior or threat directed towards another dog, animal, or person. Aggression usually involves snapping, growling, lunging, snarling, nipping, and biting.

Whenever a dog is exhibiting aggressive behaviors, the first thing that comes to mind is seeking professional help – consulting a certified dog trainer or behaviorist. However, aggression can be triggered by both behavioral and medical reasons.

 Therefore, when a dog starts behaving aggressively, it is advisable to see the vet first. If the vet rules out the underlying medical issue's presence, the next step would be seeing a canine trainer or behaviorist.

The Medical Causes of Aggression in Dogs

There are many medical causes of aggression in dogs, from teeth problems and joint pain to endocrine issues and neurological abnormalities. Some medical causes are genetic, and others are acquired.

In dogs with aggression caused by genetic conditions, the chances of curing the aggression are minimal. Instead of healing, you should put the focus on managing the unwanted behavior when it presents. On the other hand, your vet can successfully treat most of the acquired aggression-causing conditions.

Chronic or Intense pain

When a dog is feeling pain, it reacts defensively. More often than not, the defensive behavior can escalate into aggressiveness. In fact, based on reports, the most common cause of sudden aggression outbreak is pain.

So, basically, every medical condition that causes pain can trigger aggressive episodes or, at least, a grumpy attitude.

 That includes hip dysplasia, arthritis, ear infections, gum disease due to tartar buildup, or something as simple as an ingrown nail or an impacted anal gland.

Imbalanced Brain Chemistry

In humans, imbalances in the brain chemistry cause obsessive-compulsive disorders and clinical depression. In dogs, they can cause aggression. 

In the brain, serotonin is responsible for neurochemical aggression control. Consequently, if the serotonin levels are out of balance, aggression is likely to occur.

Low Thyroid Levels

Hypothyroidism is a common endocrine condition that develops when the thyroid gland does not produce an average thyroid hormone level. Hypothyroidism is not an all-or-none type of medical condition. 

Namely, in addition to normal and hypothyroid dogs, there are dogs whose thyroid gland does not produce enough thyroid hormones for normal functioning, but they are technically considered normal (closer to the lower end of the range).

Hypothyroidism is a complex condition that manifests with excessive lethargy, increased body weight, hair loss, and low cold tolerance. Dogs with hypothyroidism are more prone to experiencing anxiety attacks and manifesting aggression episodes.

Low Calcium Levels

Hypocalcemia is the general medical term for low calcium levels. Low calcium levels are most likely to occur in female dogs around three weeks after giving birth. 

The medical term for low calcium levels in nursing females is eclampsia. It happens if the dog does not receive enough calcium, in which case she starts using her own reserves to keep up with the calcium needs for milk production.

A female dog with eclampsia will exhibit aggressiveness towards people or her pups. Additionally, the nursing mother may exhibit restlessness, muscle spasms, stiff and painful gait, trouble walking, or even seizures.

Low Glucose Levels

Hypoglycemia is the medical term for low blood sugar levels. Sugar can be perceived as brain food. Without adequate blood sugar levels, the brain cannot function properly.

Small and toy breed dogs are at higher risk of developing hypoglycemia. A dog going through a hypoglycemic episode will exhibit the following signs and symptoms: weakness, moodiness, staring, glassy eyes, dazed look, staggering, aggressiveness, and in more severe cases, collapse.

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Head Trauma

Head trauma and injury are frequently accompanied by brain swelling and bleeding. These consequences tend to interfere with the brain’s normal functioning.

 Based on which portion of the brain is affected, the clinical manifestation varies. In some cases, aggression can develop after head trauma.

Brain Tumors

Brain tumors are linked with temperament changes. These temperament changes can be constant or appearing in episodes. 

The most commonly observed temperament changes include confusion, disorientation, aggression, irritability, hyper-excitability, irregular sleep habits, changes in mental status, exaggerated gait, abnormal postures, increased vocalization, visual deficits, house soiling, and appetite changes.


Hydrocephalus is a congenital malformation that occurs when the brain ventricles (fluid-filled spaces) enlarge, which causes the surrounding brain tissue to shrink. 

In human medicine, the popular term for hydrocephalus is “water on the brain”. Hydrocephalus is more common in toy and brachycephalic dog breeds.

Milder cases are often asymptomatic, while in severe cases, neurological signs are common. One of the frequently exhibited neurological symptoms is aggression.

Brain Inflammation

Encephalitis is the medical term for brain inflammation, which can be caused by bacterial or viral agents. 

Based on its nature, the encephalitis can be either acute or chronic. Acute encephalitis is more common among puppies and young dogs, while chronic encephalitis is more common in older dogs.

The inflamed brain cannot work correctly. As a result, a plethora of neurological signs occurs, including sudden aggression episodes.


Epilepsy is an increasingly common disorder among dogs. In the immediate post-seizure phase, some dogs can exhibit unusual behaviors before returning to normal. One of those unusual behaviors is aggressiveness.

At this point, the dog is not aware of what it is doing, and it should be handled carefully and with extra caution.

Behavioral Seizures

Behavioral seizures are popularly known as "rage syndrome". They occur when a partial seizure affects the brain's portions responsible for aggression control (such as the hypothalamus and limbic system). As a result, the dog exhibits sudden, haphazard, and sometimes even violent aggression for trivial or no reason.

Prior to the seizure (aggression episode), there is an evident mood change, and after the aggression episode, the dog is lethargic, depressed, and unresponsive to commands. The episode can last anywhere between a few minutes to a few days.

Chesapeake Bay Retrievers, Poodles, Bull Terriers, Golden Retrievers, Cocker Spaniels, and Springer Spaniels are likely to experience behavioral seizures.

Concluding Thoughts

Sadly, each year, thousands of dogs end up abandoned or in shelters due to aggressive behavior. Before giving up on your dog, make sure a medical condition is not the culprit for aggressiveness.

Many of the aggression-causing medical conditions can be successfully treated or at least managed. In a nutshell, before making a decision, have your trusted vet perform a thorough examination.

About the Author

Dr. Ivana Crnec is a graduate of the University Sv. Kliment Ohridski’s Faculty of Veterinary Medicine in Bitola, Republic of Macedonia.


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