There are a number of reasons why your dog may be drinking more than usual. Some of them are more serious than others. Fortunately, almost all of them can be treated once you get the right diagnosis (note: the following list is not exhaustive but does cover the most common causes of excessive water-drinking in dogs)
Vomiting or Diarrhea
Whether it’s due to parasites, or just getting into the garbage, dogs with vomiting and diarrhea will lose a lot of excess water. It is normal, therefore, for them to try to drink more to make up for these losses. If drinking more is accompanied by vomiting and/or diarrhea, it will usually resolve once the underlying gastrointestinal upset is treated.
The kidneys are responsible for filtering, concentrating, and producing urine. If kidney function is compromised, the urine produced will always be dilute. When dogs are producing nothing but watery, dilute urine, they drink more to try to compensate. You will notice large volumes of pee on walks, and sometimes accidents in the house as well.
Over time, they will not be able to keep up with the water loss and become chronically dehydrated. Also, because the kidneys aren’t excreting waste products as they should, they build up in your dog’s bloodstream. This combination of factors leads to dogs that feel unwell (listless, decreased appetite, vomiting, etc).
In most cases, kidney disease is a chronic condition of senior dogs. However, acute/sudden kidney problems can occur with infections like leptospirosis and ingestion of certain toxins like antifreeze.
Dogs with a simple bladder infection will sometimes show excessive water-drinking as one of their symptoms. The main symptom tends to be urinating small amounts more frequently. This is most common in young female dogs and is usually easy to treat with antibiotics.
Uterine Infection (Pyometra)
Female dogs who have not been spayed are at high risk of developing uterine infections (also known as a pyometra). It occurs most commonly in older dogs 1-2 months following a heat cycle. Drinking and peeing excessively is a common symptom. Other signs include fever, lethargy, and inappetance. In some cases, purulent discharge (pus) can be seen coming out of the vagina.
This is a serious and life-threatening condition, the treatment for which almost always involves surgery to remove the infected uterus.
Cushing’s syndrome (or Cushing’s disease) is a hormonal disorder where the body produces excessive amounts of cortisol. It affects primarily middle-aged and older small breed dogs. The signs tend to come on fairly gradually. The most common symptom that owners notice is excessive thirst and urination. Over time other symptoms like increased appetite, excessive panting, a pot-belly, and hair loss may occur.
Cushing’s syndrome cannot be cured completely, but many dogs can be well managed on medication that brings their drinking and peeing back to more normal levels.
Classically, the first symptom that owners will notice in their diabetic dogs is drinking and peeing excessively. An increased appetite is also noted. Left untreated, diabetes can progress to cause weight loss and severe, life-threatening illness. Fortunately, treatment with insulin and close monitoring can help to manage this disease in dogs.
Treatment with Certain Drugs
Certain medications can cause increased thirst and urination in dogs. The most common one of these would be steroids. Examples of steroids include prednisone and dexamethasone. They are used to treat a number of conditions ranging from allergies to immune-mediated disease.
The anti-seizure medication phenobarbital will also cause increased drinking and peeing. As will a medication called furosemide which is most often prescribed for dogs with congestive heart disease.
Unfortunately, cancer has to be on the list in a dog who is drinking excessively. While there are many other plausible causes, a senior pet with no other explanation for polydipsia should be evaluated to rule out an underlying cancerous cause.
A diet that is high in sodium can cause increased thirst in your dog. This could be due to the introduction of a new especially salty treat. Also, certain veterinary prescription diets are designed to be high in sodium (for example, those used to dissolve and prevent bladder stones).
In some cases, excessive water drinking can actually be a behavioral problem rather than a medical one. This is most commonly seen in new puppies who are simply enjoying playing in their water too much.