What causes cold sores in the corner of my mouth?

Herpes virus: Can cause fever blisters. Cold sore at the corner of mouth could be due to dry air, .

Cold sore???: Cold sores in the corner of your mouth is an unusual area. Not impossible but unusual. Other sores in this area can be a vitamin deficiency, or a drooling problem while sleeping at night. Or an overclosure problem if you wear dentures.

Herpes simplex virus: Cold sores are a re-activation of latent herpes simplex virus infection, at the corners of the mouth or elsewhere. Not all sores at the corners of the mouth are cold sores, though; more commonly, cracking or oozing of the skin here represents "angular cheilitis"--the result of frequent exposure to saliva, and may become secondarily infected by bacteria or yeast. See your dentist for more info.

Fungal infection: Cracking and chapping in the corners of the mouth is usually not cold sores, but is a condition called angular cheilitis, due to a fungal (yeast) infection called candida. It's common in denture wearers and others where the bite is over closed and where drooling tends to occur. An antifungal cream, nystatin, is used and the denture may be soaked in Nystatin solution. A new denture may be needed.