Why?
Dogs can technically drink small amounts of cow milk but they lack sufficient lactase enzyme to fully digest lactose. This commonly leads to diarrhea, vomiting, and digestive upset.
How much can dogs have?
A tablespoon as a very occasional treat is unlikely to cause problems in non-lactose intolerant dogs.
What to watch out for
Watch for diarrhea, gas, or vomiting after giving milk — these indicate lactose intolerance. Dogs who react badly should never have dairy products.