If you have bad breath, you've probably tried every remedy in the book at this point, from gargling mouthwash to swearing off onions. But, as it turns out, many of the causes of bad breath are not what we'd think. While diet, lifestyle and hygiene do have quite a lot to do with it, they're not the only three reasons an estimated 50 million people in the U.S. have bad breath.

"In around 90 percent of cases, bad breath is caused by a buildup of harmful bacteria under the gums and (in rarer cases) around the tongue," explains Ron Baise, DDS, founder and head dentist of 92 Dental in London. "This bacteria (often called 'biofilm' due to its ability to form a sticky layer when in large quantities) breaks down the proteins that it is attached to and, as a result, very small and gradual rotting of the outer layers of flesh that make up your gums leads to bad breath." Unlucky for us humans, as well as pretty much all of our furry friends, this bacteria thrives in warm, wet and well oxygenated environments, making our mouths the perfect breeding ground.

While the best ways to avoid bad breath are, of course, to brush twice daily and floss at least once a day (paired with regular trips to the dentist, of course), it still helps to understand what might be causing your bad breath, especially if you're religious about your oral care. Keep reading for some of the most common — yet also unexpected — causes of bad breath.

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Believe it or not, but certain prescribed medications — including those for high blood pressure, depression, nasal congestion, acne, asthma, obesity and more — can cause bad breath. The reason for this is that many medications come with the side effect of dry mouth, which Jon Marashi, DDS, California-based celebrity dentist, explains, can in turn cause bad breath. "Consulting with your physician about dosage and coming up with ways to counter attack dryness should help," he adds.

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If your diet is particularly heavy in protein, namely red meat, it might actually contribute to bad breath. "Since bad breath is caused by the breakdown of protein in your mouth, it can also be caused by an excess of a similar breakdown in your stomach that occurs when you eat a lot of protein," explains Dr. Baise. "As red meat contains more sulphur than any other protein source, it is particularly likely to cause bad breath, as more hydrogen sulphide is produced when it breaks down." This also explains the phenomenon of "keto breath," or bad breath that is associated with those who follow the ketogenic diet, which generally calls for high fats and low carbs.

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While this one might be quite obvious, it's more than worth a mention. "Not brushing or flossing your teeth and gums regularly causes the buildup of tiny food particles and bacteria in the mouth, which causes gums to become inflamed and red," explains Dr. Marashi. "This irritation can release a bad odor in the mouth signaling an infection, most likely periodontal gum disease." Visiting your dentist for a cleaning, coupled with regular brushing and flossing should help fight this.

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Oftentimes bad breath can be caused by bacterial growth or infections in your sinuses. If you have sinusitis, for example, a common condition that can be triggered by a cold or allergies, bad breath is likely to come as a side effect. "Post nasal drip in addition to sinusitis can especially cause bad breath as the encourage build-up of microbes, foreign objects and metabolites that cause bad breath," adds Tim Stirnman, DDS, dentist at Compassionate Dental Care in Lake in the Hills, Illinois. So get those sinus issues taken care of — and your bad breath might just clear up too.

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