Why a 5 kg puppy is especially vulnerable
Small breeds and young puppies process fluids differently than adult dogs, making them far more susceptible to water intoxication. Their kidneys can't rapidly excrete large volumes, so the water dilutes their blood sodium to dangerous levels. Even if your puppy seems fine now, the delayed effects of hyponatremia can appear hours later when you least expect them.
The overnight timeframe complicates things further—without supervision, you can't know how much they drank or when symptoms might start. Stagnant water often contains algae or bacteria that worsen the crisis by irritating the stomach lining, adding vomiting or diarrhea to the risks.
What silent hyponatremia looks like in puppies
Before obvious wobbliness or seizures, subtle signs like excessive drooling, restlessness, or refusing treats may appear. A puppy might seem "sleepy" when they're actually disoriented from brain swelling. Press gently on their gums—if the pink color takes over a second to return after you lift your finger, circulation is already affected.
Puppies often deteriorate faster than adults because their higher metabolic rate accelerates fluid shifts. Don't wait for visible distress; at this size, every minute counts for preventing irreversible neurological damage from cellular swelling.
Why you can't treat this at home
Attempting to balance electrolytes with salty foods or restricting water abruptly can trigger deadly fluid shifts in the brain. Veterinarians use carefully controlled IV drips to raise sodium levels at the precise rate needed—too fast causes osmotic demyelination, too slow risks seizures.
Your vet will likely run bloodwork to check sodium, potassium, and kidney values before starting treatment. They may administer diuretics or anticonvulsants depending on the results, which is why driving straight to the clinic beats waiting for a callback.
The critical first 12 hours post-exposure
Even if emergency treatment stabilizes your puppy, monitoring continues for at least half a day. Some dogs rebound quickly only to develop secondary complications like aspiration pneumonia from earlier vomiting. Keep them in a quiet, dim space during recovery—bright lights and noise worsen neurological symptoms.
Expect follow-up blood tests 6-12 hours later to ensure sodium levels remain stable. Your vet may recommend a temporary low-sodium diet and restricted water access for the next few days while the kidneys fully recover.
The step-by-step response, from the first minute to the vet visit, is in our Water Intoxication (Hyponatraemia) emergency guide.