Cat Suddenly Lethargic and Weak: Causes & Treatment

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Sudden lethargy in cats is a common and serious symptom of illness in cats. Read on for a simple, clear explanation about why this can happen, and what you should do if your cat is affected.

Quick Overview: Cat Suddenly Lethargic and Weak

clock Urgency: Very urgent; a prompt review by your vet is important.
home Home Remedies: Home remedies should only be considered once your veterinarian has confirmed there is not a serious underlying condition with your cat.
injection-syringe Vaccine Available: No specific vaccine is available, although vaccines can be given against some of the causes of sudden lethargy, such as viral infections.
jam-medical Treatment Options: Supportive care is important, including keeping a cat warm, comfortable, and well hydrated. However, a specific diagnosis of the reason for the lethargy needs to be obtained, so that specific treatment can be given.
medical-files Diagnosis: Sudden lethargy can be diagnosed by simply observing a cat, but the important task for your vet is to make a specific diagnosis of the underlying cause of the lethargy, which involves careful history taking, a thorough physical exam, and often laboratory tests and other investigations.
search Common Symptoms: Sudden change in behavior. Does not want to walk around, play, or jump on furniture. Wants to stay, lying down, in one place. Might hide away. Does not react normally when you speak to them or try to interact with them.

What Is Sudden Lethargy?


A cat experiencing sudden lethargy will lie around much of the day and show little interest in playing.

Sudden lethargy is a situation where a cat suddenly loses energy and interest in life. The definition of lethargy is “a lack of energy and enthusiasm,” or to use a medical definition, lethargy is “a pathological state of sleepiness or deep unresponsiveness and inactivity.” When a cat develops this condition, it’s a clear sign that something is seriously amiss.

Cats that are lethargic seem depressed and very tired, sleeping far more than normal, and not doing much else. They may also appear weak. Lethargic cats may not want to get up to use the litter box and can act sick in other ways, as well.

Also Read: Why Do Cats Sleep So Much?

Lethargy in cats can have varying degrees, including:

  • Mild lethargy: The cat isn’t as interested in playing or being as active normal.
  • Moderate lethargy: The cat seems very tired and disinterested, and won’t play or engage with you at all.
  • Severe lethargy: The cat doesn’t even want to move and may refuse to lift up their head.

How Common Is Sudden Lethargy?

Sudden lethargy in cats is uncommon, which is why it is sign that something is amiss.

Sudden lethargy does not develop commonly, but when it happens, it can be worrying. Lethargy is not a specific disease; it can be caused by a long list of possible underlying causes (see below).

Symptoms Of Sudden Lethargy In Cats

A cat experiencing sudden lethargy might stop grooming themselves, so the coat may appear unkempt.

A lethargic cat stops behaving normally. They do not want to walk around, play, or jump up on furniture. They just want to stay, lying down, in one place. They do not react normally when you speak to them or try to interact with them. As well as this altered behavior, affected cats normally also have a significantly reduced appetite, refusing to eat normally.

Many lethargic cats will hide in a quiet, dark place, such as under the bed or in a closet. They tend to prefer areas away from people and other pets.

Importantly, lethargy is different to being tired or sleepy. Lethargic cats they will not wake up and return to normal if you call their name and engage with them. They stay profoundly sleepy, unresponsive, and inactive. In the longer term, they may suffer other signs including weakness and weight loss, depending on the underlying cause.

Other symptoms or signs that may be noticed at the same time include panting, coughing, sneezing, weight changes, increased thirst, vomiting, or diarrhea.

Also Read: 10 Subtle Signs Your Cat May Be Sick

Causes Of Sudden Lethargy In Cats

Mild Lethargy

Mild lethargy, where a cat seems less active than normal, could be caused by stress or a medical issue.

Mild lethargy can be caused by stress and anxiety, or it can be seen as an early sign of a more serious medical problem.

A cat can become stressed or anxious for many different reasons. The cat might be nervous about a change in the house or environment, they might be ill or injured, or there might be a lot going on around the house. Any number of other things could cause stress or anxiety for a cat, including the loss of a close pet or an owner, moving homes, the arrival of a new pet or family member (such as a new baby), or when a cat goes to a new owner.

To help a stressed or anxious cat, think of what might have changed around the house that could be upsetting to your cat. Provide the cat with plenty of enrichment, such as toys, laser, playtime, scratching posts, cat trees, cat grass, etc. Try veterinary-recommended cat-specific pheromones or calming supplements. Remove any sources of stress that can be eliminated.

Also Read: How To Pet A Cat – The 3 Basic Do’s And Don’t’s

Moderate Or Severe Lethargy

will cats fake a limp for sympathy

Moderate lethargy, where a cat lies around and won’t play or engage, is usually caused by illness or injury.

There is a long list of possible causes of sudden moderate or severe lethargy, including:

  1. Pyrexia, or a high temperature. This is probably the most common cause of sudden lethargy. It most commonly happens when the body’s immune system is reacting to the presence of bacteria (e.g., from an abscess) or viruses. This is part of the body’s natural defense system. Sometimes the same reaction can be seen for one or two days after vaccination, in which case the slightly elevated temperature can be a normal response and may not be anything to be unduly worried about as long as the cat has no other signs of illness.
  2. Pain. Many people mistakenly believe that cats always vocalize when in pain. This is not true at all—often, lethargy is the main sign that a cat is in pain. There are many possible causes of pain, from trauma (e.g., an obvious external wound), to internal damage (dental disease, slipped disks in the spine, blood clots, twisted organs, etc).
  3. Dehydration. Dehydration causes a sense of fatigue and dullness, so any disease that causes dehydration can result in lethargy (e.g., vomiting, diarrhea, kidney disease, etc.).
  4. Metabolic crises. From low blood sugar (hypoglycemia) to reduced red blood cell count (anemia), some internal conditions can lead to reduced glucose or oxygen-carrying ability of the blood, and the result is sudden lethargy. High blood sugar (such as that caused by diabetes mellitus) can lead to the production of toxic by products (e.g., ketones) that can also make a cat very lethargic. Hepatic lipidosis (fatty liver) is another example of disruption to the metabolism causing lethargy-inducing toxins.
  5. Parasites. Fleas, ticks, and worms can disrupt a cat’s metabolism in a range of different ways that can cause lack of energy and the resulting lethargy.
  6. Medication. Many drugs can have side effects that may include lethargy, from anti-seizure medication to anti-anxiety drugs and many others.
  7. Poison. A wide range of poisons can cause lethargy, sometimes due to a direct neurological effect, and sometimes because of a general metabolic toxicity.
  8. Heart disease. When the heart stops working properly, the blood can no longer be pumped around the body effectively, depriving the muscles and organs of glucose, oxygen and other essential products, resulting in lethargy. There are many possible causes of heart disease, from heartworm infection to cardiomyopathy to hyperthyroidism.
  9. Respiratory disease. If a cat is unable to breathe properly, the blood may not be oxygenated well enough, and the muscles, tissues, and organs may be deprived of oxygen, leading to lethargy.
  10. Liver disease and kidney disease. The liver and kidneys process toxins and waste products from the metabolism, so when they are not working properly, these will accumulate in the blood stream, causing the cat to feel unwell. This can result in lethargy.
  11. Cancer. Cancer can have multiple effects in the body, from pain to the production of toxins to alterations in the bloodstream, with the overall impact causing lethargy.
  12. Gastrointestinal disorders such as vomiting and diarrhea can cause lethargy due to dehydration, but also due to the lack of normal absorption of energy-giving nutrients.
  13. Urinary tract disease. As well as direct kidney disease, conditions affecting the bladder and lower urinary tract can cause pain, discomfort, and secondary kidney disease, resulting in lethargy. Examples include urinary tract infections, cystitis, and urinary obstruction, with the latter more common in male cats. It is important to carefully observe the litter box of any cat that is lethargic so you can monitor their urinary output.
  14. Miscellaneous inflammatory or immune type diseases. When the immune system is overloaded or malfunctioning, lethargy can be one of the signs.
  15. Neurological disease. If the brain is not working properly, lethargy is a common sign that there is a problem.

Also Read: Why Is My Cat Peeing A Lot?

Diagnosing Sudden Moderate Or Severe Lethargy

When you take a cat suffering from sudden lethargy to your veterinarian, the following steps may be taken.

1. Detailed History Taking

You veterinarian will ask you many questions about your cat to try to pinpoint the cause of the lethargy.

Your vet will discuss every aspect of your cat’s condition and overall health care. There are many possible causes of sudden lethargy, and this history will help to differentiate the various possible causes. Examples include possible exposures to toxins, medications they may have been given, foods they may have been eaten, history of parasite control, etc.

2. Physical Examination

Your veterinarian will take your cat’s temperature to check for fever.

Your vet will check your cat over carefully, taking their temperature, carrying out auscultation of your cat’s chest (listening with a stethoscope), feeling your cat’s body all over (including checking for enlarged lymph nodes), checking for any painful areas or swellings, watching your cat move around, and generally examining your cat to look for any abnormalities. As part of the physical exam, they may weigh your cat, checking for weight loss (or in some cases, weight gain).

3. Routine Laboratory Tests

Blood and urine tests can reveal what’s going on with your cat’s health.

It’s very likely that your veterinarian may carry out standard blood work, including the usual panel of diagnostic tests, such as hematology (blood count) and biochemistry profiles. A urine sample will also often be checked. These tests are often carried out in the veterinary clinic laboratory, so results are often available after only a short period of time.

4. Specialized Blood Tests

Your veterinarian may recommend specific blood tests for some viral infections such as feline leukemia virus (FeLV) and feline immunodeficiency virus (FIV), since there are significant implications if your cat is positive for either of these. Other specialized tests may include measuring thyroid hormone levels.

5. Other Tests

X-rays may be recommended based on the results of your cat’s physical exam or lab tests.

Radiography (x-rays), ultrasound, and blood pressure measurement may all be suggested as ways of finding out more about what’s going on inside your cat.

Your veterinarian will advise you on which detailed investigations are needed.

How Much Does It Cost To Treat A Cat With Sudden Lethargy?

Ask your veterinarian for an estimate of charges for the diagnostics and treatments they recommend.

It is impossible to estimate the cost of treating lethargy, as there are so many possible factors going on in the background of individual cases. You should ask your veterinarian for a detailed estimate before agreeing to proceed with treatment.

Costs could vary from $100 for a simple underlying cause (e.g., a harmless reaction to a vaccine) to $4,000 or more for an exceptional case of sudden lethargy when there are complex underlying causes.

Also Read: How To Introduce A New Kitten To An Older Cat

Treatment For Sudden Moderate Or Severe Lethargy

Depending on what’s causing your cat’s lethargy, your vet might prescribe medications or other treatments.

Treatment for sudden lethargy depends entirely on the underlying cause which is why it is so important that your veterinarian makes an accurate diagnosis. Your veterinarian will then advise you on the appropriate treatment for your individual cat, which might include:

  • Antimicrobial medication (broad spectrum antibiotics) to kill bacteria are likely to be needed if the cat has a bacterial infection.
  • Anti inflammatory pain relief (e.g., non-steroidal anti inflammatory drugs, or NSAIDS) might be given in some cases.
  • Intravenous fluid therapy might be given if your cat is suffering from a number of conditions, including dehydration.
  • As well as general supportive care, if there are specific underlying causes such as parasites, low blood glucose, organ failure, poisoning, etc., then a number of different treatments might be suggested.
  • Some specific causes (e.g., abscesses) may require surgical intervention (e.g., draining the abscess).

Monitoring And Prognosis

It is likely that your veterinarian will ask you to bring your cat back for frequent hands-on physical re-examinations until the lethargy has fully resolved.

Final Thoughts

Person petting a cat who has been sick

Sudden lethargy is a sign that your cat may have a serious underlying condition, and prompt veterinary attention should be sought to ensure that they can be treated early and effectively.

Also Read: When Will My Cat Stop Hissing at New Kitten?

Frequently Asked Questions

Why would a cat suddenly become lethargic?

Sudden lethargy in cats can be caused by many different causes, from infections to pain to poisonings to organ failure and many others.

How do I know if my cat has sudden lethargy?

The main signs, as listed above, are sleepiness, dullness, and reduced activity compared to normal.

Is lethargy in cats an emergency?

This should never be ignored as it has the potential to be caused by a serious and life-threatening underlying cause, requiring urgent and comprehensive veterinary treatment.

Why is my cat suddenly lethargic and not eating?

There is a long list of possible causes, some of which are very serious. To be safe, you should take your cat to your local veterinarian as soon as possible so that the underlying cause can be established.

What to give a cat that is lethargic?

You should keep a lethargic cat warm and comfortable, offering small amounts of tasty food and fresh water. However you do need to seek out prompt veterinary care, as there are no specific home treatments available. It is very important that your vet makes a diagnosis of the underlying cause so that the correct treatment can be given as soon as possible.

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About Dr. Pete Wedderburn, DVM

Dr Pete Wedderburn qualified as a vet from Edinburgh in 1985 and has run his own 4-veterinarian companion animal practice in County Wicklow, Ireland, since 1991. Pete is well known as a media veterinarian with regular national tv, radio and newspaper slots, including a weekly column in the Daily Telegraph since 2007. Pete is known as "Pete the Vet" on his busy Facebook, Instagram and Twitter pages, regularly posting information on topical subjects and real-life cases from his clinic. He also write a regular blog at His latest book: “Pet Subjects”, was published by Aurum Press in 2017.