If your dog drinks excessively, it could be because he is suffering one of the following. Be acutely aware when they do this.
Pain, stress, heat exhaustion (dehydration), illness (vomiting and dehydration), kidney failure or disease.
Pain and Stress
These cause an increase in heart rate and respiration which in turn causes the body temperature to rise. The dog pants to cool down and has to drink a lot to keep its mouth wet so evaporation can take place. Also a full stomach has a feeling of contentment that physiologically eases the dogs discomfort. If you can not see an obvious reason for this take your dog to the vet ASAP
You can assist your dog to cool down by bringing it into your bath and standing it in cold water, sponge the dog down especially the head, groin and armpit areas.
The skin is thin and less hair helps the blood cool faster.
The toe pads are also a heat release area. If it does not cause more stress and discomforts try to lay the dog down into the water in a 'drop' position, not a flank (side) position.
Keep drinking water available but restrict it in intervals so the dog does not gorge it and then vomit it back again.
You have to think for your dog. The dog is under stress and does not know any better.
Keep offering water to drink, as the dog becomes more comfortable. Also use a cooling fan on the dog. If the condition of the dog is not improving wet it down all over and ring your vet for any further advice then take to the Vet ASAP.
Do not wrap up! As it will prevent heat evaporation!
If your dog does improve allow time for it to stabilize, before taking your vet for a check up. Always make sure where ever your dog is, that it has adequate water supply and air flow at all times. Cars are deadly for dogs in summer, they are mobile ovens even with the windows down. A shady tree is a much safer idea.
Often a dog will drink when it has Vomiting, Diarrhea, or any general gastric illness. The dog does not know when to stop and that excessive amounts are detrimental to its recovery. Veterinary attention may be needed. If the dog is vomiting and drinking you must take the water away until the vomiting ceases. If the dog becomes worse seek veterinary attention immediately. Try gradually re-introducing small amounts of water again and fast the dog from all food for 24hrs.
Feed only small bland meals after that.
Acute (sudden) or Chronic (gradual) kidney failure or disease can cause a change in the dogs drinking habits depending on how toxic (sick) the dog is. Sometimes they do drink excessively but also they may not drink at all. Drinking in excess will also cause an increase in urinating.
The dog also shows signs of lethargy, vomiting, dehydration, poor appetite, a rough harsh coat, bad breath and anemia.
Veterinary attention is needed. Blood tests and Urine tests, will help show up the problem. Treatment involves replacing lost body fluids and electrolytes also antibiotics, drugs to stop vomiting, anabolic steroids for recovery, weight gain and repair of the kidney damage and vitamins.
Monitoring of blood and urine levels has to be done until the dog stabilizes.
Your vet will then explain about how to care for the kidney-damaged dog. Dietary changes, medicines will help ease the strain on your dogs' kidneys. These are avoiding meat, eggs, cheese and bones.
Prescription diets are the safest and easiest way to do this. They are available in cans from your vet. This will help long life, it will probably require more veterinary treatment as the kidneys worsen. Ultimately euthanasia is the nicest option as kidney transplants are not readily available to dogs and owner finances.