Cold sores are, by definition, always caused by the herpes simplex virus, usually HSV-1, so when a true cold sore appears on the lip or around the mouth, it is a form of oral herpes. Sores in the same area that are not caused by this virus are not considered cold sores.
Cold sores usually start with tingling, burning, or itching, followed by small fluid-filled blisters that break and form a crust. Outbreaks tend to come and go, often after a cold or flu, periods of stress, or a weakened immune system.
The virus spreads easily through close contact, especially kissing, and by sharing items like cups, utensils, towels, or lip balm, and it can sometimes spread even when there is no visible sore. During an outbreak, it is important to avoid these behaviors and to wash hands after touching the affected area.
If mouth sores are frequent, very painful, or do not look like typical cold sores, the person should be assessed by a primary care doctor or dermatologist, who may order tests or prescribe cold sore medicine when needed.