Why Is My Cat Suddenly Aggressive? Causes and What to Do

By Tailwisp Editorial TeamUpdated October 20, 20264 min read

A cat that has been gentle for years suddenly hisses, swats or bites. It is alarming, and it is a signal. Cats rarely become aggressive without a reason. Finding the reason is the first step, and the first stop is usually the vet.

Medical causes

Pain and illness are leading causes of sudden aggression.

• Pain, from arthritis, dental disease or injuries. See arthritis in cats, dental disease in cats and signs of pain in cats.
• Hyperthyroidism, which can cause irritability. See hyperthyroidism in cats.
• High blood pressure, or neurological disease.
• Cognitive dysfunction in older cats. See caring for a cat with dementia.
• Vision or hearing loss, which makes cats startle.
• Infections and other illnesses.

Behavioral causes

• Fear. Cornered or startled cats lash out.
• Redirected aggression. A cat aroused by something it cannot reach, such as an outdoor cat, may attack a person or pet nearby. See redirected aggression in cats.
• Play aggression. See play aggression in cats.
• Territorial or status aggression, toward other cats. See aggression between cats.
• Overstimulation, during petting. See why does my cat bite me while petting.
• Stress, from a move, new pet or noise. See signs of stress in cats.
• Maternal aggression, in a mother with kittens.
• Frustration, from boredom or lack of outlets.

First steps

1. Stay safe. Do not touch or approach an angry cat. Use a door, a thick blanket or a towel to separate.
2. Give space, and keep the room quiet.
3. Note the details: where, when and what happened before.
4. See a vet, for a full exam, and bloodwork if advised. See cat wellness exam: what to expect.
5. Keep a diary of incidents.

What not to do

• Do not yell, hit or spray the cat.
• Do not stare it down.
• Do not corner the cat.
• Do not pick up a riled cat.
• Do not ignore the first signs.

Prevention and management

• Identify and reduce triggers, such as outside cats seen through windows.
• Provide hiding places, perches and enough resources.
• Keep routines, and use play for outlets. See how to play with your cat.
• Reward calm behavior.
• Consider pheromone diffusers. See do cat pheromone diffusers work.
• Re-introduce cats slowly after a fight. See how to separate fighting cats.

Bites and scratches

Wash any bite wounds well and seek medical care, because cat bites can become infected quickly. Tell your doctor about the bite.

When to call a behaviorist

If aggression continues after a vet check, or is severe, ask for a referral to a veterinary behaviorist. Medication may be part of the plan.

Kittens and seniors

Young cats may be rough in play. Senior cats may be irritable due to pain. See senior cat care guide.

Types of aggression and how to spot them

Fear aggression shows with flattened ears, dilated pupils, a crouched body and hissing. Redirected aggression appears out of the blue, often after the cat has seen something that upset it. Pain aggression happens when you touch a sore area. Play aggression has silent, bouncy, ambush-style behavior. Territorial aggression is usually directed at other cats or visitors. Knowing the type helps you and your vet choose the right solution.

Keeping everyone safe

If a cat is in the middle of an attack, do not use your hands. Throw a blanket over it, or use a barrier such as a pillow to separate. Leave the room, and close the door. Allow the cat several hours to calm down, because arousal can last. After a redirected episode, keep the cat and the target apart for a while, then reintroduce slowly with treats and play.

What your vet might do

Your vet will examine the cat, check the mouth, joints and eyes, and may recommend bloodwork, a blood pressure check and a urine test. If there is no medical cause, they may suggest a behavior plan or refer you to a veterinary behaviorist. Some cats benefit from medication that reduces anxiety, always used alongside training and environmental changes.

Progress takes time

Calming a cat that has become aggressive can take weeks or months. Keep a record, celebrate improvements and avoid situations that you know trigger the cat. Many cats return to normal once the cause is found and treated.

When to see a vet

See a vet promptly for any sudden aggression. Use the US vet directory to find a clinic.