The short answer on milk for dogs
Milk is not poisonous to dogs, but it genuinely upsets most adult dogs' stomachs, and the upset is predictable rather than bad luck. A puppy nursing from his mother produces plenty of lactase, the enzyme that splits lactose, the sugar in milk. After weaning, that enzyme production falls away in most dogs, because milk is no longer part of the natural diet. When an adult dog then drinks a bowl of cow's milk, much of the lactose passes through undigested, drawing water into the bowel and feeding gas-producing bacteria on the way. The usual result, within hours, is flatulence, gurgling, loose stool, or a bout of diarrhoea that resolves on its own but leaves the dog feeling miserable.
Why some dogs seem fine with it
Tolerance varies dog by dog, which is why one household swears milk is harmless while another cleans the carpet after a single splash in the food bowl. Dogs who kept some lactase production into adulthood can digest a modest amount, and the amount matters enormously: a lick from a cereal bowl is a very different load from a full saucer. Fermented dairy is a separate case — plain yoghurt and hard cheese carry far less lactose because the fermenting bacteria consume much of it, so a dog who cannot handle milk may manage a spoonful of yoghurt without symptoms. That difference is about the lactose content of the food, not about dairy being universally safe or unsafe.
A sensible way to offer it, if you offer it at all
If your dog loves the taste and you want to know where he stands, test small and once: a tablespoon or two of plain milk for a medium dog, less for a small one, given on its own rather than mixed into a meal so you can read the result clearly. Then watch the next twenty-four hours of stool, appetite, and comfort before deciding milk has passed. Skip flavoured milks entirely — chocolate milk adds cocoa, which dogs must never have, and sweetened versions add sugar a dog does not need. Milk should also never stand in for water, and a dog who is already prone to loose stools, or who is recovering from a stomach upset, should not be experimented on at all.
The signs that say milk was a mistake
One soft stool after a first taste is information, not an emergency; go back to the normal diet, keep water available, and let the gut settle. Repeated vomiting, diarrhoea that continues into a second day, blood in the stool, a swollen or painful belly, or a dog who stops eating and seems flat are a different matter — at that point contact your veterinarian and mention exactly how much milk he drank and when. Puppies bought from a breeder are sometimes still being given milk by new owners out of kindness; an orphaned or very young puppy who genuinely needs milk needs a proper puppy milk replacer, not cow's milk, and your vet can point you to feeding amounts for his weight.
Keeping the verdict where feeders will see it
Whether milk suits your dog is exactly the kind of small, hard-won fact that gets lost between family members, so one person repeats the experiment another already ran. Write the outcome down with the feeding routine instead of relying on memory. In KinsLe, Care Teams lets you record that milk is off your dog's menu — or that a spoonful is fine — right beside his meals, so every person who feeds him works from the same tested verdict.