What supplements/vitamins help prevent cold sores?

None - No deficiency. This is a symptom of herpes simplex virus. Avoid contact w/ body fluids of infected folk. (kissing, eating utensils, towels, razors, etc.) once infected, there's no sure way to prevent cold sores. But you can reduce outbreaks and spreading it. Wash hands often & avoid touching sores to prevent spread to eyes, genitals or to others. Avoid triggers (colds, flu, stress, sunlight on face). Sorry.