My 5-year-old dog's breath is awful — I always thought dog breath was just normal?

It isn't normal. Healthy dog breath is fairly neutral, and the smell you are describing almost always comes from dental disease under the gumline — and that disease hurts, even though he is still eating.

Dogs almost never stop eating because of mouth pain. They chew on the side that hurts less and swallow food whole, so appetite tells you nothing. The part of the problem that actually matters sits below the gum, where you cannot see it, which is why a real assessment means an examination under anaesthetic with dental x-rays. Everything visible above the gumline is only the part you happen to be able to see.

Before the appointment: lift his lip on both sides and photograph the back teeth, because that is usually where it starts. Watch which side he chews on. Look for a red line where the gum meets the tooth, a tooth that is darker than its neighbours, or food dropping out of his mouth. That is genuinely useful information to bring in.

A sudden very foul smell together with drooling, blood, or pawing at the mouth is a different story — that points at a fractured tooth, a mass, or something wedged across the roof of the mouth, and it should be seen soon. And breath that smells sweet, like nail polish remover, or like urine or ammonia, is not a dental problem at all: those are metabolic signs and need a vet promptly.

Please do not scrape anything at home, and be wary of anaesthetic-free dental cleaning. It polishes the surface you can see and leaves the disease that is causing the smell exactly where it is. Your vet will look at his mouth, his age and the x-rays together and tell you what actually needs doing.