Why does My Dog Drink from the Toilet? – When presented with a choice, it makes sense that dogs and other canines would prefer fresh water over stagnant water.
Through natural selection, those who didn’t make the distinction were probably killed off by organisms that bloom in stagnant water. In our homes, cool, fresh water is readily available in toilet bowls. To a dog’s sensibilities (though adorable, still quite different from ours), this is as attractive a choice as any, particularly if the behavior is reinforced by quenched thirst over the years.
To discourage toilet-drinking, water bowls should be washed and refreshed at least twice daily.
It helps also to consider whether your dog likes what the water bowl is made from. To learn his preference, offer choices of stainless steel, ceramic and plastic.
If offered his choice of bowls filled with fresh, cold water, chances are your dog will forget the toilet.