One of the most common types of mouth sore is the cold sore or fever blister. These small, blister-like lesions are caused by the herpes simplex virus (different from the virus that causes genital herpes). Cold sores can be difficult to identify in their earlier stages because they are not visible. But once the sores become visible, they are easier to identify. Cold sores go through five distinct phases and each phase has different identifying characteristics. Inspect your mouth for the symptoms of each stage to know whether or not you have a cold sore.
Feel for a tingling sensation. In the earliest stages of the herpes simplex virus, before the cold sore blister is visible, you will notice a burning or tingling sensation on your mouth, usually on the edge of your lip. This tingling area can also begin to feel painful, tight and slightly swollen as the cold sore starts to develop.
Inspect your lips for a blister-like lesion. Once the cold sore has become visible, it will appear like a fluid-filled blister. Cold sores are usually on the lips, but can also occur on the face near the mouth. Cold sores can develop as individual blisters or a cluster of blisters in one area.
Look for signs of leaking fluid. Soon after a cold sore appears, it will begin "weeping" or leaking clear or yellowish fluid. This is usually the most painful stage of the virus. The blister will feel raw and sore and have a burning sensation.
Feel the blister for signs of crusting. Once the cold sore has finished the leaking phase, it will begin to crust over, forming a scab. The scabbing phase usually is accompanied by an itching or burning sensation. Sometimes, the scab becomes irrated and bleeds a little.
Look for signs of healing. A cold sore usually only lasts about one week to 10 days before it begins to heal. The scab will slowly start to flake away, revealing clear skin beneath the dry, dead skin. Cold sores usually do not cause long-term scars.