Why might your dog suddenly become lethargic?

Why might your dog suddenly become lethargic?

Health & Safety

Just like humans and all other animals too, dogs have distinctive moods and temperaments that can vary considerably depending on how they are feeling. When it comes to things like activity levels, exercise and generally how interested and engaged your dog is in what is going on around them, your dog’s energy levels and tolerance can vary from day to day, and throughout the day too.

For instance, a dog that has just had a lively walk will often want to sleep or chill out for some time afterwards, which makes perfect sense-but if your dog appears to be flat and lethargic for no good reason and this is out of character, this may indicate that something is amiss, which requires investigation.

In this article, we will look at some of the most common of the numerous potential causes of lethargy in dogs, and how to identify what is amiss. Read on to learn more.

Identifying lethargy in dogs

Lethargy is generally fairly self-evident, and can be identified fairly easily in dogs. A lethargic dog will generally be slower to respond to a call or stimulus than a dog that is switched on and alert, and they will often spend a lot of time lazing about and sleeping.

They may show less interest than normal in treats and food, and may be reluctant to get up, go walking, or interact with you. When you know your dog, it is of course much easier to spot out of character behaviour or changes in them, and of course, what is out of character lethargy for one dog may simply be the normal activity levels for another!

In the next section, we will look at some of the most common causes of lethargy in dogs.


What you feed to your dog affects virtually everything about them, including their energy levels and even temperament! If your dog is not fed enough to support their lifestyle, or their food is not a good match for their activity levels, this can of course make them lethargic and a bit flat.

If your dog has recently started becoming more active or you have changed their food, this is something to look at.


Weight can of course have a large effect on how active and lively your dog is, and being overweight will have a knock-on effect on your dog and cause them to be less active and less able to exercise.

However, if you put your dog on a diet and so, reduce their calorie intake, this too can make them less lively than normal while they adjust to the change!

Infection and illness

If your dog has picked up a cold or another contagious condition that is going around, they may become rather flat and lethargic for a couple of days before the other symptoms become apparent or acute, as their bodies start to prepare to fight off the illness.


Anaemia is a disorder of the red blood cells and iron, which can lead to lethargy, lack of tolerance for exercise, and a range of other issues such as a tendency to fainting. If your dog’s mucous membranes in the eyes or gums are very pale, anaemia may be the cause.

Respiratory problems

If your dog has allergies, asthma or an upper respiratory tract infection, this can cause intolerance to exercise because your dog won’t be able to get enough air or breathe freely. This can be the case in brachycephalic breeds of dog in particular, as they are more prone to respiratory problems and overheating.

Medication side-effects

If your dog is prescribed medication for another health condition, read the instruction leaflet that comes with it carefully so that you can make yourself aware of the potential side effects that can accompany the condition.

Lethargy is a common side effect of many medications, particularly during the first couple of days during which they are prescribed it, which will often rectify itself within a day or so. Speak to your vet if you have any concerns, or if the problem is pronounced or acute.


Heatstroke poses a significant risk to dogs during the hotter months of the year, and while it is entirely preventable, it can develop quickly and be very serious.

If the weather is very hot and your dog faints, won’t move, is too hot or has been exercising hard, work to get their temperature down as soon as possible, and contact your vet for further advice.

Autoimmune disorders

Autoimmune disorders can lead to lethargy and general flat, dull reactions from your dog, so ask your vet to investigate this and maybe run some tests.

Poisoning or toxicity

The different effects that poisons and toxins can have on your dog can be very varied, and can take anything from a few minutes to a few days to show symptoms. If your dog shows any other symptoms of poisoning or if you know that they have ingested or come into contact with something dangerous, speak to your vet as a matter of urgency to check it out.

CO poisoning

Finally, a thankfully rare but often fatal cause of unexplained lethargy in dogs is carbon monoxide poisoning, which is also a risk to humans as well. For more information on carbon monoxide poisoning in dogs, check out our prior article.

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