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I wouldn't muzzle him - and that wouldn't stop him barking anyway. One of my other dogs who recently died, age almost 18, suffered from dementia, but that hadn't occurred to me with this one. He's very rickety now and is on medication because he keeps falling over, but he still behaves like a puppy and always has, so other than the night barking I've not noticed any difference in his behaviour. He saw the vet a couple of weeks ago, but thanks for the suggestion. I'll speak to him again today.