Why Has My Pet Suddenly Become Aggressive? A Vet Explains The Hidden Causes
Why Has My Pet Suddenly Become Aggressive? A Vet Explains The Hidden Causes
Written by Alessandro Didiano (doctor in veterinary medicine, MRCVS)
Disclaimer: This article is personally written by me, a licensed veterinarian with more than a decade of clinical experience. It’s for general information only and not a substitute for professional veterinary advice. Always consult a vet for health concerns or emergencies.
"I am very worried", said the lady. "Usually he is very aggressive, and now he stopped hating everybody".
I looked at the patient, Richie, a very docile 11-year-old boxer. He approached me and licked my hand.
"This is not a bad thing", I said.
"Doctor, trust me, there is something wrong with him. Richie would be growling at you right now. it is not normal for him to lick your hand".
"What is she complaining about?", I arrogantly thought to myself.
Two days later I called the owner, and I gave her the news that Richie's MRI of the brain sadly showed a large tumor affecting his left frontal lobe. "I knew there was something wrong with him", was all she said, sobbing.
Despite being very much loved by his current owner, Richie had a very rough start in life, and always showed high aggression levels to people and other dogs. However, his loving owner was able to manage this perfectly, and keep him and the public safe.
When Richie suddenly started to be docile and friendly, she knew something was not right.
Richie's story is quite incredible, but it is one in a million. Usually, the complaint is the opposite: a docile animal becomes aggressive.
As a clinical veterinarian, I often hear the complaint, “My pet has become aggressive” A once-friendly dog starts growling, or a gentle cat suddenly lashes out. Obviously, this behavior can be alarming and confusing.
Sudden aggression in previously docile pets is a serious red flag, one that shouldn’t be ignored. It can be a sign of pain, illness, or another underlying medical condition, including neurological disorders. In this article, I’ll break down the hidden causes of sudden aggression and what you can do to help your pet.
1. Pain: The #1 Cause of Sudden Aggression
If your pet has never shown aggression before, pain is definitely the first thing to consider. Animals instinctively hide pain (especially cats, but this is true also for dogs), so aggression may be their way of saying, “I am sore, don’t touch me!”
Common Pain-Related Causes of Aggression:
Arthritis – Older pets with sore joints may react aggressively when touched. Back or neck pain- Pets with spinal pain, for example caused by a slip disc, can easily snap when touched on the painful area.Abdominal pain: Gastroenteritis and pancreatitis are the two most common causes of abdominal pain. These are usually (but not always) associated with gastrointestinal signs, such as vomiting or diarrhea. Dental Disease – This can be one of the most severe forms of pain. Ear Infections – Dogs and cats with ear infections may growl or snap if their head is touched. Injuries – Sprains, fractures, wounds, bruises. These can all cause pain and therefore aggression.
What to Do: If your pet is suddenly aggressive, the first essential thing to do is to see your vet for a physical examination, to determine any painful areas. Often clients think they can do this without the assistance of a vet, but believe me: it is not easy for a non-professional to detect abdominal or spinal pain.
2. Neurological Disorders: When The Brain Triggers Aggression
Aggression (or, in Richie's case, the lack of aggression!) can also be caused by neurological conditions affecting the brain. These cases are often a medical emergency and require urgent veterinary attention.
Neurological Causes of Aggression:
Brain Tumors – Especially in older pets (but sometimes also seen in younger animals), tumors can cause confusion and aggression.Encephalitis: Similarly to brain tumor, an inflammatory-infectious process affecting the brain can cause aggression. This condition can be seen in animals of every age. Seizures & Epilepsy – Some pets experience post-seizure aggression due to disorientation. Rabies – Though rare, rabies ((at the time of writing, the UK is rabies-free, but the disease is still occasionally seen in the US) can cause aggression. Cognitive Dysfunction Syndrome (Pet Dementia) – Similarly to people suffering from Alzheimer's, senior pets with cognitive decline may become aggressive. What to Do: If your pet shows sudden aggression along with other neurological symptoms (staring into an empty space, pacing, seizures, disorientation, or personality changes) a neurological exam is essential.3. Fear-Based Aggression: A Fight-or-Flight Response
When a pet feels threatened or overwhelmed, aggression can be a defensive reaction. This can happen due to several reasons, the most common of which are:
New environments – Moving house, loud noises, or unfamiliar places. Negative experiences – This is extremely common after a vet visit or because of harsh training methods (which should never be used) Poor socialization – Lack of early exposure to different people, pets, and sounds can make normal experiences feel threatening for your pet. What to Do: Always avoid punishing your pet, as this can worsen fear. Instead, use positive reinforcement and, if needed, consult with a trainer or vet behaviorist. When introducing your pet to a new environment, try to make changes as gradually as possible, providing familiar items (for example their bed or their favorite toys) and creating a safe space where they can retreat when they feel stressed. Calming pheromones or classical music have also been shown to reduce stress. Finally, if poor socialization is an issue, try to expose your pet to new experiences at their own pace, and always use treats and plenty of praise to reinforce positive interactions. Always start with low-stress situations and gradually (and slowly) increase exposure, ensuring each new experience is associated with something positive ( in order to build confidence over time).4. Hormonal Imbalances: When the Body Affects Behavior
Certain medical conditions are well known to change your pet’s hormonal balance and mood, causing uncharacteristic aggression.
Hyperthyroidism (Common in Cats, Very Rare In Dogs) – High thyroid hormone levels can cause irritability and, in some cases, aggression. Cushing’s Disease or Diabetes (Relatively Common In Dogs, Very Rare in Cats) – This is a condition caused by excessive cortisol production, and it can result from a tumor in the adrenal glands (situated in the abdomen, next to the kidneys) or the pituitary gland (situated in the brain). It affects energy levels and behavioral changes, such as irritability and anxiety. Sex Hormones – Unneutered pets may be more territorial or aggressive. However, it is very important to debunk a myth here: aggression caused by fear, anxiety, or poor training will not automatically go away after neutering the animal. In these cases, extensive behavior therapy with a certified pet behaviorist is often needed, and there are even causes where neutering an animal can make aggression worse, especially in male animals (testosterone can provide confidence. Some dogs with fear-based aggression may become more anxious and reactive after neutering). What to Do: A blood test can check for hormonal imbalances, which are often treatable with medication.5. Territorial & Resource Guarding: Protecting What’s Theirs
Your pet may suddenly growl, snap, or bite when you approach their:
Food bowl – “This is my food, just stay away!” Favorite spot – This can be a bed, the couch, or even a whole room. Toys or bones – As above: "This is mine, don't touch it" A person – Some pets become can become possessive over their owners. Once I met a 40-year-old lady who admitted to me that she accepted not to have a romantic partner and possibly a family because her dog wouldn't allow anyone to get near her. What to Do: In these cases, you have to work with a professional trainer to desensitize your pet and prevent resource-guarding aggression.6. Cognitive Decline: When Senior Pets Change Personality
Older pets can develop Cognitive Dysfunction Syndrome (CDS), which, as mentioned above, is similar to dementia in humans. Confusion, anxiety, and nighttime restlessness can lead to uncharacteristic aggression. This is typically seen in pets above the age of 11-12 years old, and the most common signs include pacing at night, forgetting familiar routines (for example, they may start toileting in inappropriate places), staring at walls and getting "lost" in familiar places.
What to Do: Special diets, supplements, and some medications can slow cognitive decline and ease anxiety.If you're looking to support your pet’s brain function, you could consider these options
Supplement for supporting brain function in older dogs – This supplement is designed to promote cognitive health in aging dogs.
Supplement for supporting brain function in older cats – Help your feline friend maintain their cognitive function with this specially formulated supplement.
These products are widely recommended by us veterinarians to help manage the symptoms of CDS in pets. Important: when considering or using supplements for Cognitive Dysfunction Syndrome, it's very important to have realistic expectations. These products can support brain function and help alleviate some symptoms of cognitive decline but, by all means, they are not a cure-all. The effectiveness of supplements may vary from pet to pet, and results are generally seen gradually over time.
7. Medication Side Effects: Could the Treatment Be the Problem?
Finally, it is important to note that some prescription medications can cause behavioral changes, including aggression.
Medications That May Trigger Aggression:
Steroids (e.g., prednisone, dexamethasone) Propalin: this is a medication commonly used to treat urinary incontinence in dogs. It is highly safe and effective, but in some cases, it can trigger irritability, restlessness and even aggression (this is, however, rare. During the course of my career, I had to stop this medication in two animals because of Propalin-induced aggression.
Anti-seizure medication: anti-seizure medications are extremely important in helping control seizure in epileptic pets, but in some cases they can caused increased anxiety or irritability, and even aggression in some pets. Certain Painkillers or sedatives can cause confusion and consequent aggression.What to do: if you notice aggression after starting a new medication, talk to your vet about alternative options.What Should You Do If Your Pet Becomes Aggressive?
It can be frightening to see your beloved and docile pet suddenly showing signs of aggression, but please try not to panic (I am aware this is easier said than done) and, most importantly, avoid punishment (It can escalate aggression. Use positive reinforcement instead.)
First of all, it is important to rule out any medical causes, as discussed above. Schedule a vet check-up for a full health evaluation.
If the vet examination doesn't find any obvious medical problem, try to observe for any triggers. Try to note when and where the aggression occurs (does it always occur when you touch them while they eat? this means they don't want to be touched while they eat?). If the trigger is simply identified and you can easily avoid it, the job is easily done.
However, unfortunately, in many cases, the situation is not so easy to fix. Please, in such cases, before someone gets seriously hurt, consider professional help . A veterinary behaviorist or trainer can help with behavioral modification.
When to See a Vet Immediately
There are cases where "waiting out" is just wrong, and the only thing you can do is to contact your local vet for an immediate appointment. These include cases where:
• The aggression appears suddenly with no clear trigger.
• Your pet shows neurological signs (seizures, confusion, staring at the empty space).
• The aggression is particularly severe or clearly worsening.
Final Thoughts: Sudden Aggression Is a Symptom, Not a Personality Change
As a vet, I always tell my clients that sudden aggression isn’t about “bad behavior”, it’s a sign that something is wrong.
Whether it’s pain, neurological disease, or fear, the key is to identify the root cause and address it early.