Differential diagnosis: top left - Canker sore (aphthous ulcer) inside the mouth, top right - Herpes labialis (Herpes 1, fever blister, cold sore), bottom left - Angular Cheilitis and bottom right - Chapped Lips. Treatment: An application of an appropriate acidic solution can be most effective in treating a Canker Sore. Anti-viral medication is typically effective in treating an outbreak of Herpes but no current medication will eliminate it from the body. Herpes is highly contagious when it is visible. Angular Cheilitis is an inflammation that occurs at the corners where the upper and lower lips join; it is frequently infected by fungus like Candida (yeast, thrush) that can also infect other parts of the body. It may be contagious. A fungal infection may be treated with Clotrimazole cream. Chapped Lips may be caused by the environment, diet, habits, an allergic reaction, or mouth-dryness associated with other medical conditions or medication. Treatment will depend upon the etiology.