What causes bad breath?: Dr Ros O' Leary, Periodontist
The most common causes are to do with oral bacteria. So whether it's inflammation of your gums, with a lot of really sort of nasty bacteria in and around. It can often come from your tongue - some people have quite rough surfaces to their tongue and they retain a lot of bacteria. It can be things like infections in your mouth, which is rarer. It can be like open decay - which is also rarer. It can sometimes be - very rarely - due to gastric reflux and problems from the stomach. But, as I said, it's more common orally.
People who smoke tend to have bad breath, people who drink a lot of coffee tend to have bad breath. So there are habits that you can do, that can also make the situation worse. But the fundamental thing is your teeth need to be clean, and that is where one would start.