Cracks or sores in the corner of the mouth can be rather annoying, like a paper cut — small, and irritating. Fancy medical terms for this condition include "angular stomatitis," "angular cheilitis," or "perleche."
A number of different things can be behind the development of such cracks:
vitamin deficiencies (vitamin B2 or riboflavin)
iron-deficiency anemia (low red blood cells due to too little iron)
allergic reaction to some substance (particularly products used on the lips, such as lip gloss/stick/balm, and even dental care/oral hygiene products, such as toothpaste)
poorly fitting dentures (or no teeth or no dentures at all)
Sometimes applying an anti-yeast cream [such as ketoconazole (brand name is Nizoral) that can be found over-the-counter (OTC) at your local drugstore] to the area will work. You can also mix it with a little dab of 1 percent hydrocortisone cream, also available OTC. Rub this mixture over the area 2 - 3 times each day. If you don't see improvement in under a week, or if the area becomes redder, more swollen, or terribly uncomfortable, you'll need to make an appointment with your health care provider or dermatologist. If you continue to have problems with cracking at the corners of your mouth, check with your health care provider to see if there are other measures you need to be taking, such as using vitamin or iron supplements, applying only petroleum jelly on the lips, switching to a milder baking soda based toothpaste, or being properly fitted with dentures.