Plus, how to fix it!

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Anybody with a face knows the agony of chapped lips. The telltale pang when you crack a smile, the peeling skin, the specks of blood. You likely don't need a dermatologist to diagnose those symptoms, and you probably know the cause: dehydration, sun exposure, cold weather or some combination of all three.

But when it's only the corner of your mouth that's red and flakey, chances are you're experiencing something else entirely. And we're not talking about cold sores.

You might be dealing with a condition called angular cheilitis. And don't panic. According to S. Manjula Jegasothy, M.D., a Miami-based dermatologist, it's so common that it's taught on day one of dermatology school.

And the cause of those pesky sores? Drool!

"Yeast is always present in the mouth and saliva, so when we drool in our sleep, it collects in the corners of the mouth, like rain in a gutter," Dr. Jegasothy explained to Refinery29. And it's this buildup of yeast that causes dry, red flakes of skin to form. "Because the grooves in the corners of our mouth get deeper as we age, this can worsen the older we get," she continued.

WATCH: Why Cold Weather Causes Lips to Chap and How You Can Stop It

Sleeping with your mouth open or regularly licking your lips can also increase your chances of coming down with angular cheilitis. And as Maya Feller, a New York-based registered dietitian and nutritionist, explained to Refinery29, so can a poor diet.

"A nutritional deficiency of B2, B3, B6, and iron can increase the risk of developing angular cheilitis," Feller noted. "High-carbohydrate diets, as well as glucose-rich diets, can also provide an environment where oral candidiasis can thrive."

If you're struggling with angular cheilitis Dr. Jegasothy has a simple, albeit surprising, remedy. "Any over-the-counter athlete's foot cream with butenafine 2%, like Lotrimin Ultra, will help," she told Refinery29. Apply it twice a day—morning and night—and wait for it to work its magic.