Most people may mistake oral herpes for other common mouth ulcers like cankers sores. The main difference is that oral herpes is caused by a virus called herpes simplex virus (either HSV-1 or HSV-2), while the cause of canker sores is mostly associated with vitamin deficiencies, allergic reactions, stress, weaker immune system and infections. Cold sores or oral herpes are highly contagious and occur on both outside and inside the mouth, around the lips and on other parts of the skin. On the other hand, canker sores occur almost always inside the mouth like on the tongue, inner cheeks, inner parts of lips, throat and other soft tissues in your mouth.