According to Dr. Starr, halitosis can be caused by food, tobacco use, gum disease, dry mouth, or even underlying medical conditions. These can all cause bacteria to be released in the mouth. See, anaerobic bacteria survive and grow where oxygen isn't present; when they break down the food you eat, this special bacteria “degrade[s] the sulfur-containing amino acids cysteine and methionine into the foul-smelling volatile sulfur compounds (VSCs),” says Dr. Starr.
And, like other noxious gases your body emits, there's a huge variety of possible smells. If you notice a sweet or alcoholic odor wafting from your mouth, for example, Dr. Starr says that could be a red flag that you’re prediabetic. So if your friends are always asking if you’ve been drinking, even though you’ve been sober for hours, it might be time for a check up.