Day 1: Prodrome (tingle) stage - Before a cold sore has formed, you may feel a tingling, itching or burning sensation beneath the skin, usually around the mouth or the base of the nose. Applying antiviral medications during this stage can help alleviate cold-sore symptoms.

Days 2 to 3: Blister stage - An outbreak of fluid-filled blisters is the first visible sign of cold-sore formation.

Day 4: Ulcer or weeping stage - Typically, the most contagious and painful stage of cold sores is when blisters rupture, leaving a shallow, reddish, open sore.

Days 5 to 8: Crusting stage - After a few days, the blisters dry up and form a yellow or brownish crust, which eventually falls off. During this stage, it is important to care for the scab, which can crack or break.

Days 9 to 12: Healing stage - Usually a series of scabs will form on the lesion, which eventually flake off. Each new scab will be smaller than the previous one, until the cold sore heals completely, usually without scarring.

Reviewed: January 2012