Why Has My Dog or Cat’s Behavior Suddenly Changed?

By Dr Duncan Houston

A pet who suddenly refuses walks, snaps when touched, urinates indoors, paces at night or hides from the family is not necessarily being difficult.

Behavior changes are often among the earliest signs of pain or illness. They can develop before vomiting, lameness, fever or another obvious physical symptom appears. Medical and behavioral problems can also occur together, which is why a new behavior should not automatically be treated as a training failure. (Merck Veterinary Manual)

The real concern is not simply that your pet is “acting strangely”. It is whether pain, neurological disease, organ dysfunction or another physical problem is changing how they feel and respond to the world.

Quick Answer

A sudden or significant behavior change in a dog or cat warrants a veterinary examination, especially when it is accompanied by changes in appetite, movement, sleep, urination, defecation or awareness.

Seek emergency care immediately if your pet collapses, has a seizure, presses their head against objects, struggles to breathe, cannot urinate, becomes severely disoriented, develops very pale gums or shows uncontrolled pain or aggression alongside neurological abnormalities. (Merck Veterinary Manual)

Behavior Changes at a Glance

Behavior change	Medical causes to consider
Refusing walks, stairs or jumping	Arthritis, spinal pain, muscle injury, weakness, heart or respiratory disease
Snapping when touched or lifted	Pain, ear disease, dental disease, abdominal discomfort, fear associated with previous pain
House soiling	Cystitis, urinary obstruction, kidney disease, diabetes, diarrhea, constipation, arthritis, cognitive decline
Pacing or panting at night	Pain, endocrine disease, cognitive dysfunction, anxiety, medication effects
Hiding or becoming unusually clingy	Pain, nausea, systemic illness, sensory decline, fear
Excessive licking or grooming	Itch, skin disease, urinary pain, abdominal pain, neuropathic discomfort
Staring, air snapping or repetitive movements	Focal seizures, gastrointestinal discomfort, neurological disease, compulsive behavior
Sudden food guarding or appetite changes	Dental pain, nausea, increased hunger, medication effects, endocrine disease or learned behavior
Increased vocalization	Pain, hyperthyroidism, hypertension, sensory loss, cognitive dysfunction
Sudden aggression	Pain, fear, neurological disease, metabolic illness, sensory decline or medication effects

These patterns help guide an investigation, but none of them confirms a diagnosis by itself. Recognized medical causes of behavioral signs include pain, neurological disorders, sensory loss, endocrine disease, gastrointestinal disease, urinary disease and organ dysfunction. (Merck Veterinary Manual)

Behavior Is a Clinical Sign, Not a Diagnosis

Behavior is one of the ways an animal responds to its internal and external environment.

When that behavior changes, the cause may be:

• Medical

• Neurological

• Behavioral

• Environmental

• A combination of several factors

For example, a cat may initially stop using the litter box because arthritis makes climbing into it painful. Even after the pain is treated, the cat may continue using the carpet because a new location preference has developed.

Similarly, a dog with an ear infection may begin growling during ear handling. After the infection resolves, the dog may still anticipate pain and remain frightened of being touched.

Treating the disease is therefore essential, but some pets also need environmental changes or behavior modification to undo what they learned while they were unwell.

Pain Is One of the Most Commonly Missed Causes

Pain does not always cause crying, limping or obvious distress.

Chronic pain often appears as a gradual change in normal behavior. Owners may notice that their pet sleeps more, becomes less sociable, hesitates before moving, wakes during the night or becomes irritable when handled. AAHA pain guidelines emphasize that owner observations at home are particularly valuable because many signs of chronic pain are subtle or absent during a short clinic examination. (AAHA)

Signs of Pain in Dogs

A painful dog may:

• Refuse walks or slow down earlier

• Hesitate before climbing stairs

• Stop jumping into the car

• Pant or pace without an obvious environmental reason

• Struggle to settle at night

• Guard a particular body area

• Become clingy or withdrawn

• Growl when approached while resting

• Become less tolerant of children or other animals

• Lose housetraining

• Lick one joint, paw or part of the body repeatedly

Signs of Pain in Cats

Cats frequently hide pain rather than displaying it openly.

Possible changes include:

• Reduced jumping

• Hesitation on stairs

• Hiding

• Reduced grooming

• Overgrooming one painful area

• Urinating beside the litter box

• Difficulty entering a high-sided tray

• Becoming less tolerant of petting

• Hissing or biting when lifted

• Sleeping in unusual places

• Reduced interaction

• A hunched or tucked-up resting posture

Feline pain guidance identifies behavioral change as the primary indicator of pain and specifically lists reduced appetite, hiding, impaired movement, decreased grooming, toileting changes and aggression among the signs owners should report. (catvets.com)

Common Sources of Hidden Pain

Pain-related behavior may result from:

• Osteoarthritis

• Spinal or neck disease

• Dental disease

• Ear infections

• Skin inflammation

• Urinary tract disease

• Gastrointestinal disease

• Injuries

• Cancer

• Postoperative pain

• Neuropathic pain

• Nail or paw disease

A pet who suddenly resists handling should be examined before being labelled stubborn, dominant or poorly trained.

Neurological Disease Can Look Behavioral

Neurological conditions can alter awareness, movement, sleep, temperament and learned behavior.

Possible neurological signs include:

• Staring into space

• Repetitive chewing or chomping

• Air snapping

• Sudden episodes of fear or aggression

• Facial twitching

• Tremors

• Circling

• Pacing

• Confusion

• Falling

• Loss of housetraining

• Abnormal sleep

• Failure to recognize familiar people

• Sudden blindness

Focal seizures may occur without collapse or full-body convulsions. They can instead produce staring, facial movement, repetitive swallowing, chomping, intermittent aggression or other brief episodes that look like unusual behavior. (Merck Veterinary Manual)

What About Fly-Biting or Air Snapping?

Fly-biting describes a dog appearing to watch and snap at something that is not visibly present.

It should not automatically be diagnosed as:

• A focal seizure

• Acid reflux

• A compulsive disorder

• Attention-seeking

In a small prospective series of seven dogs, gastrointestinal abnormalities were identified in all seven, and several dogs improved after the gastrointestinal disease was treated. One dog also had Chiari-like malformation. Because the study was very small, it shows that gastrointestinal disease belongs on the differential list, not that every air-snapping dog has reflux. (PubMed Central (PMC))

A useful investigation may include video review, physical and neurological examination, blood and urine testing, eye assessment, gastrointestinal evaluation and, in selected patients, advanced neurological testing.

Gastrointestinal Disease Can Change Behavior

Nausea, reflux, abdominal pain and altered gut motility can make an animal restless, withdrawn or unusually reactive.

Possible behavior changes include:

• Lip licking

• Repeated swallowing

• Licking floors or furniture

• Air snapping

• Pacing after meals

• Stretching or assuming unusual postures

• Eating grass

• Refusing food

• Sudden food selectivity

• Guarding food

• Waking during the night

• Eating non-food objects

• Seeking constant attention

• Hiding after eating

These behaviors are not specific to gastrointestinal disease. They may also occur with anxiety, dental pain, seizures or learned behavior, so the entire clinical pattern matters. Gastrointestinal disease is nevertheless recognized as a potential cause of licking, pica, restlessness, abnormal sleep and fecal house soiling. (Merck Veterinary Manual)

Repeated vomiting, abdominal distension, severe pain, retching without producing vomit or profound weakness requires urgent assessment.

Urinary Disease Often Appears as a Behavior Problem

Urinary disease can cause:

• House soiling

• Frequent trips outside

• Repeated litter-box visits

• Restlessness

• Genital licking

• Vocalization

• Irritability

• Aggression when lifted

• Night waking

• Hiding

• Reduced appetite

A previously housetrained dog may begin having accidents because urine production has increased, bladder control has declined or urination has become painful.

A cat may avoid the litter box because it has become associated with cystitis pain. Arthritis may also make the tray difficult to enter, while kidney disease and diabetes can produce more urine than the cat can comfortably hold.

Every cat that newly begins house soiling should receive a physical examination and urinalysis. Further tests may include urine culture, blood tests, radiographs or ultrasound, depending on the findings. (catvets.com)

A Male Cat Straining Without Passing Urine Is an Emergency

Repeated attempts to urinate with little or no urine can indicate urethral obstruction.

Other warning signs include:

• Crying in the litter box

• Repeated genital licking

• Vomiting

• Loss of appetite

• Weakness

• A painful abdomen

• Collapse

Do not assume the cat is constipated or behaving strangely. Attend an emergency veterinary service immediately.

Skin Disease and Itching Change Mood and Sleep

Itch is profoundly uncomfortable.

A dog or cat with chronic skin disease may:

• Become irritable

• Sleep poorly

• Avoid touch

• Overgroom

• Chew or scratch repeatedly

• Chase the tail

• Rub against furniture

• Hide

• Guard parts of the body

• Become less playful

• React aggressively during grooming

Possible causes include parasites, allergies, bacterial or fungal infection, ear disease, painful wounds and neuropathic discomfort.

In cats, apparent “psychogenic” overgrooming or hyperesthesia must be diagnosed cautiously. Pain, itch, skin disease and neurological disorders need to be excluded before the behavior is classified as compulsive. (Merck Veterinary Manual)

Endocrine and Metabolic Disease Can Alter Personality

Hormonal and metabolic disorders may affect appetite, sleep, activity, elimination and temperament.

Hyperthyroidism in Cats

Hyperthyroid cats may develop:

• Increased activity

• Night waking

• Restlessness

• Increased vocalization

• Irritability

• Increased appetite

• Weight loss

• Urine marking

• Aggression

Hypothyroidism in Dogs

Dogs with hypothyroidism may show:

• Lethargy

• Reduced responsiveness

• Exercise intolerance

• Weight gain

• Skin changes

• Irritability in some cases

The relationship between thyroid disease and aggression is complex. Thyroid testing may be appropriate when the history and physical signs support it, but a thyroid panel is not automatically required for every behavior consultation.

Hyperadrenocorticism

Dogs with Cushing’s disease may show:

• Persistent panting

• Increased appetite

• Increased thirst

• Night waking

• House soiling

• Restlessness

• Irritability

Diabetes and Kidney Disease

Increased thirst and urination can lead to:

• Indoor accidents

• Night waking

• Restlessness

• Repeated requests to go outside

• Litter-box overflow

• Reduced tolerance of normal routines

Recognized medical associations between endocrine disease and behavior include hyperactivity and irritability with feline hyperthyroidism, lethargy with canine hypothyroidism, and panting, anxiety or house soiling with hyperadrenocorticism and diabetes. (Merck Veterinary Manual)

Liver and Kidney Disease Can Affect the Brain

Severe liver or kidney dysfunction may allow metabolic waste products to affect normal brain function.

Hepatic encephalopathy may cause:

• Intermittent lethargy

• Disorientation

• Personality change

• Aimless wandering

• Pacing

• Head pressing

• Circling

• Drooling

• Blindness

• Aggression

• Ataxia

• Seizures

• Coma

These episodes may fluctuate, sometimes making the pet appear normal between events. Advanced signs such as head pressing, circling, seizures or reduced responsiveness require emergency treatment. (Merck Veterinary Manual)

Do not independently place a pet on a severely protein-restricted diet because neurological signs appeared after eating. Dietary management depends on the exact liver disorder and must still provide adequate nutrition.

Sensory Loss Can Cause Fear and Aggression

Loss of hearing or vision changes how an animal interprets its environment.

A pet may:

• Startle when touched

• Sleep more deeply

• Bark or vocalize unexpectedly

• Appear to ignore cues

• Become reluctant to go outside

• Hesitate in dim lighting

• Collide with objects

• Become defensive when approached

• Soil indoors

• Appear confused in familiar places

A deaf or visually impaired animal may snap because someone appeared suddenly within its personal space, not because its underlying personality has changed.

Approach these pets gently, create predictable routines and use vibration, scent, touch or visual cues appropriate to the senses they retain.

Cognitive Dysfunction in Senior Pets

Age-related cognitive dysfunction is well recognized in dogs and also occurs in cats.

Common changes are summarized by the acronym DISHAA:

• Disorientation

• Altered social Interactions

• Changes in Sleep-wake cycles

• Loss of Housetraining or learned behaviors

• Altered Activity

• Increased Anxiety

Dogs may sleep more during the day, pace at night, become lost in familiar rooms, forget training or interact less with family members. Cats may become more vocal, restless or socially demanding. (AAHA)

Cognitive dysfunction is a diagnosis of exclusion. Pain, sensory decline, brain disease, hypertension, kidney disease, endocrine disorders and other organ dysfunction can create very similar signs. A thorough history, physical examination, neurological assessment, blood and urine testing and, in selected cases, brain imaging may be required. (AAHA)

Early recognition matters because management tends to be more useful before the condition becomes advanced.

Medication Can Affect Behavior

Medication side effects or interactions may contribute to:

• Sedation

• Poor coordination

• Increased appetite

• Increased thirst

• Panting

• Restlessness

• Sleep disruption

• Reduced inhibition

• Confusion

• Irritability

• Urinary accidents

Always tell your veterinarian about:

• New prescriptions

• Dose changes

• Supplements

• Flea and tick products

• Over-the-counter medications

• Human medications the pet may have accessed

Do not stop anticonvulsants, corticosteroids, antidepressants or other long-term medication abruptly unless directed. Drug adverse effects and contraindications are recognized contributors to behavior problems, particularly in older patients receiving several medications. (Merck Veterinary Manual)

How Worried Should You Be?

Lower Risk

Your pet has:

• A mild isolated change

• Normal appetite and drinking

• Normal breathing

• Normal urination and defecation

• No pain

• No neurological signs

• Normal movement

• Improvement within several hours

What it may mean: A temporary environmental or emotional response is possible.

What to do: Monitor closely for 24 hours and record what happened. Arrange a veterinary appointment if the behavior returns, persists or is unusual for that individual pet.

Moderate Concern

Your pet has:

• A change lasting more than 24 to 48 hours

• Reduced play or activity

• Mild appetite change

• New house soiling

• Night waking

• Repeated hiding

• Reluctance to jump or walk

• Increased sensitivity to handling

What it may mean: Pain, urinary disease, gastrointestinal discomfort, endocrine illness or another medical condition may be contributing.

What to do: Arrange a veterinary examination within the next few days.

High Concern

Your pet has:

• Sudden aggression

• Significant pain

• Refusal to eat

• Recurrent vomiting

• Marked mobility change

• Repeated abnormal episodes

• Rapid weight loss

• Severe restlessness

• New confusion

• Urinary difficulty

• A dramatic personality change

What it may mean: A significant medical or neurological condition is possible.

What to do: Seek same-day veterinary care. Keep children and other animals away if the pet is reacting defensively.

Critical

Your pet has:

• Collapse

• Seizures

• Head pressing

• Severe disorientation

• Sudden blindness

• Inability to stand

• Open-mouth or labored breathing

• Blue, grey or very pale gums

• Inability to urinate

• Severe abdominal pain

• Suspected poisoning

• Aggression accompanied by altered awareness or neurological abnormalities

What it may mean: A life-threatening neurological, respiratory, urinary, metabolic or circulatory emergency may be present.

What to do: Attend an emergency veterinary hospital immediately.

When Is This an Emergency?

Seek immediate veterinary care when a behavior change is accompanied by:

• Collapse or near-collapse

• A seizure lasting more than a few minutes

• Repeated seizures

• Head pressing

• Circling with altered awareness

• Severe weakness

• Sudden inability to walk

• Open-mouth breathing

• Blue, grey or white gums

• Repeated straining without passing urine

• Severe abdominal distension

• Uncontrolled pain

• Suspected toxin exposure

• Major trauma

• Sudden blindness

• Extreme agitation followed by reduced responsiveness

Do not attempt to discipline, restrain or provoke a neurologically abnormal or severely painful animal. Contact the emergency clinic while travelling so the team can prepare for safe handling.

What To Do Right Now

1. Keep Everyone Safe

Pain and confusion can make even a gentle pet defensive.

Avoid:

• Hugging or lifting the pet unnecessarily

• Touching a guarded area

• Cornering them

• Allowing children to approach

• Attempting to physically prove that they can still walk

• Punishing growling, hissing or avoidance

Use doors, gates and quiet rooms to create distance safely.

2. Take a Video

Record the behavior when this can be done without provoking it.

A video may help distinguish:

• A seizure from compulsive behavior

• Pain from fear

• Ataxia from reluctance

• Coughing from retching

• Vomiting from regurgitation

• Urinary straining from constipation

Do not recreate a dangerous episode merely to obtain footage.

3. Start a Behavior and Health Log

Record:

• Date and time

• What happened immediately beforehand

• Duration

• Whether the pet responded to their name

• Appetite

• Water intake

• Urination

• Stool

• Vomiting

• Sleep

• Mobility

• Medication

• Recovery afterwards

Patterns around meals, exercise, medication or particular times of day can be diagnostically valuable.

4. Check Basic Function

Ask:

• Is the pet eating?

• Are they drinking more or less?

• Are they urinating normally?

• Are they passing stool?

• Can they walk comfortably?

• Are they breathing normally?

• Do they recognize familiar people?

• Can they settle and sleep?

• Are their gums a normal pink colour?

A behavior problem accompanied by loss of basic function deserves faster assessment.

5. Arrange a Veterinary Examination

Tell the clinic that the appointment concerns a sudden behavior change.

Bring:

• Videos

• Your written log

• Medication and supplement list

• Details of diet changes

• Previous medical records

• Information about recent injuries, travel or toxin exposure

6. Do Not Give Human Medication

Human pain relief and sedatives can be toxic or may conceal signs needed for diagnosis.

Do not give medication prescribed for another pet.

How Will the Veterinarian Investigate the Change?

The investigation should be guided by the specific behavior, age and physical findings.

History

The veterinarian will ask:

• When did the behavior begin?

• Was the change sudden or gradual?

• Is it constant or episodic?

• What triggers it?

• How long does recovery take?

• Have appetite, thirst or toileting changed?

• Has medication changed?

• Has the household or routine changed?

• Is the pet painful during movement or touch?

Physical Examination

The examination may include:

• Observation of posture and gait

• Orthopedic assessment

• Neurological examination

• Oral and dental examination

• Ear examination

• Skin assessment

• Eye and vision assessment

• Abdominal palpation

• Cardiovascular examination

• Pain assessment

• Blood-pressure measurement

A complete physical and neurological examination is considered a fundamental part of investigating behavior problems before they are classified as primarily behavioral. (Merck Veterinary Manual)

Diagnostic Tests

Depending on the case, testing may include:

• Complete blood count

• Biochemistry

• Urinalysis

• Urine culture

• Blood glucose

• Electrolytes

• Liver and kidney testing

• Thyroid testing when indicated

• Blood pressure

• Radiographs

• Ultrasound

• Joint or spinal imaging

• MRI or CT

• Cerebrospinal fluid analysis

• Gastrointestinal investigation

• A structured pain-treatment trial

Not every pet needs every test. The examination and history should determine which investigations are most likely to provide useful answers.

Will the Behavior Return to Normal?

That depends on the cause, how long it has been present and whether the behavior has become learned.

A dog with dental pain may become sociable again soon after appropriate dental treatment.

A cat with arthritis may need pain management and a lower-sided litter box before house soiling improves.

A dog that was repeatedly hurt during ear medication may remain afraid of handling after the infection resolves and require gradual cooperative-care training.

A senior pet with cognitive dysfunction may not return completely to previous behavior, but early treatment, pain management, environmental enrichment and a predictable routine can improve comfort and function. (AAHA)

The best outcomes usually come from treating both components:

1. The medical condition that started or worsened the behavior

2. Any fear, habit or environmental pattern that now maintains it

Common Mistakes

Assuming the Pet Is Being Defiant

A pet that refuses stairs, handling or toileting routines may be painful or physically unable to comply.

Punishing House Soiling or Aggression

Punishment adds fear and may suppress warning signals without treating the underlying disease.

Blaming Everything on Old Age

Age increases the likelihood of disease. It does not make pain, night waking, confusion or incontinence unimportant.

Ordering One Test and Stopping

Normal routine bloodwork does not rule out arthritis, focal seizures, dental disease, urinary pain or structural brain disease.

Assuming Fly-Biting Is Always a Seizure

Gastrointestinal, neurological, visual and compulsive causes must all be considered.

Starting Behavior Medication Before Investigating Pain

Anxiety and pain can coexist, but sedation or behavioral medication should not replace a medical examination.

Ignoring New Medication

A dose change, drug interaction or adverse effect may explain the timing of the behavior change.

How Can Medical Behavior Changes Be Detected Earlier?

Know Your Pet’s Normal

Pay attention to:

• Sleep

• Appetite

• Water intake

• Play

• Walking speed

• Jumping

• Grooming

• Toileting

• Social interaction

• Response to handling

Small changes become easier to recognize when you know the baseline.

Record Body Weight

Unexplained weight loss or gain may provide an earlier clue than behavior alone.

Monitor Mobility

Take occasional videos of your pet:

• Walking

• Climbing stairs

• Jumping

• Rising from rest

• Entering the litter box

These create useful comparisons when gradual changes occur.

Maintain Dental and Preventive Care

Dental pain, ear disease, parasites, obesity and untreated arthritis can all alter behavior.

Review Senior Pets More Frequently

Senior pets benefit from regular examinations that actively assess pain, cognition, sensory function, mobility, blood pressure and organ health. Early recognition of cognitive dysfunction and chronic pain can improve the response to treatment. (AAHA)

Keep the Environment Accessible

For pets with reduced mobility or vision:

• Add non-slip flooring

• Use ramps or steps

• Provide low-entry litter boxes

• Keep furniture layouts stable

• Improve night lighting

• Place food and water within easy reach

• Provide supportive bedding

These changes reduce frustration while the underlying condition is being managed.

Frequently Asked Questions

Can pain make a friendly pet aggressive?

Yes. Pain can lower tolerance for touch and make a pet react defensively when approached, lifted or moved. Sudden aggression should prompt a pain and medical assessment before being treated as a training issue. (Merck Veterinary Manual)

Why has my housetrained pet started having accidents?

Possible causes include urinary disease, increased urine production, diarrhea, constipation, pain, impaired mobility, sensory decline and cognitive dysfunction. Medical causes should be excluded before retraining begins. (catvets.com)

Can thyroid disease change behavior?

Yes. Hyperthyroidism in cats can cause increased activity, night waking and irritability. Hypothyroidism in dogs can contribute to lethargy and reduced responsiveness. Testing should be based on the full clinical picture rather than ordered automatically for every behavior change. (Merck Veterinary Manual)

Can a seizure look like strange behavior?

Yes. Focal seizures may cause staring, chomping, facial movements, air snapping, sudden fear, intermittent aggression or repetitive behavior without full-body convulsions. (Merck Veterinary Manual)

When should I see a veterinary behaviorist?

A veterinary behaviorist is particularly useful when medical problems have been treated or excluded but fear, aggression, compulsive behavior or anxiety remains. Medical and behavioral treatment can also proceed together when both are clearly contributing.

Final Thoughts

Behavior changes are often communication from a pet who cannot explain what feels different.

The most important points are:

1. Sudden behavior change should be treated as medical until proven otherwise.

2. Pain is commonly expressed through withdrawal, restlessness, house soiling or aggression.

3. Neurological disease can resemble anxiety, disobedience or compulsive behavior.

4. Urinary, gastrointestinal, skin, endocrine and organ disease can all alter behavior.

5. Sensory decline may make a previously confident pet frightened or defensive.

6. Cognitive dysfunction is a diagnosis of exclusion, not an automatic explanation for every senior-pet change.

7. Medical and behavioral causes frequently occur together.

8. Videos and written logs can be as valuable as verbal descriptions.

9. Punishment can worsen fear and obscure important clinical signs.

10. Collapse, seizures, head pressing, breathing difficulty or inability to urinate requires immediate care.

The mistake that causes the most harm is asking, “How do I stop this behavior?” before asking, “Why has this behavior started?”

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ASK A VET™ can help you organise videos, timelines, appetite changes, toileting patterns and medication history while you arrange direct care with your veterinarian. A pet with seizures, collapse, severe pain, breathing difficulty, head pressing or inability to urinate still requires immediate hands-on emergency treatment.