Likely cause: Bacteria.
What to do: Your first line of defense is pretty obvious: Brush your teeth (and tongue) at least twice daily, and floss every night to get rid of trapped food particles that can rot and leave an unpleasant odor. But if stinky breath continues, talk to your doc. The smell could stem from an infection in your stomach caused by Helicobacter pylori (the bacteria responsible for ulcers). Your doc can do a urea breath or a stool test to check for H. pylori, which requires antibiotics. An overgrowth of other bacteria in your gut can also raise a stink. "A probiotic can help repopulate your intestines with good bacteria," says Dr. Roizen. "An over-the-counter pill often works better than yogurt or other food sources." Bad breath is also sometimes caused by sinus problems or liver or kidney problems.