What's the difference between cold sores and an allergic reaction ?

A lot...: Cold sores are typically due to viral infection, specifically herpes simplex virus (hsv). Cold sores are generally self-limiting and resolve on their own. An allergic reaction is an immune system response due to allergen exposure. Some patients may have a contact dermatitis around the mouth in response to allergen contact that may look like cold sores. Your physician can help you sort this out.

Depends on cause: Aphthous stomatitis involving the common canker sore can occur in up to 15% of americans, and may involve some bacteria like staph epidermidis. However, similar-appearing aphthous ulcers can be seen with herpes, crohn's disease, and other systemic disorders, if you have multiple aphthous ulcers or frequently recurring ones please have the lesions checked out.