To us, toilet water is gross and full of germs. But to dogs, toilet water is refreshing. The toilet gets flushed at least five times a day, or more, depending on how many people live in the house. The constant flushing oxygenates the water, and the porcelain bowl keeps the water cool.
Dogs don't understand what humans use the toilet for. And even if they did, they probably wouldn't stop drinking from the toilet. After all, many dogs do eat poop, so the toilet water isn't going to gross them out.
So how can you stop your dog from drinking from the toilet? Well, you need to make the water in his water bowl more appealing. Wash the water bowl and fill it with fresh water at least twice during the day. You could also keep the toilet lid down or try keeping the bathroom door closed.
You might also want to try putting out water in different types of bowls to see whether your dog has a preference. He may find the water in a plastic bowl unappealing, but like the water in a ceramic bowl. You can also try stainless steel, though glass is probably not a good idea.
Another technique you can try is purchasing a pet fountain. Fountains continuously circulate and oxygenate water, so it stays fresh throughout the day. Many dogs (and cats) prefer to drink from these fountains than from stagnant water bowls.
If you're having trouble breaking your dog's toilet drinking habit, should you be concerned that the toilet water will harm him? Depending on the cleanliness of the toilet tank and bowl, toilet water is actually pretty clean, and dogs' digestive systems are much better than ours at killing harmful bacteria, so the chances of your dog getting sick from germs in the toilet are low.
Toilet cleaners, however, are potentially toxic to dogs. So if there is any chance your dog will drink from the toilet, be sure to flush the toilet thoroughly after scrubbing it.