Typically, either a fungal infection (the top culprit is Candida ... your dermatologist for treatment options. A less common cause of the cracks in the corner of your mouth is a dietary deficiency.
Mix 1 tsp. of baking soda in a cup of warm water, swirl it in your mouth for 30 seconds and then spit it out, he suggests. Repeat this two or three times a day to help treat a Candida infection.