Cold sores, or fever blisters, in and around the mouth are caused by herpes simplex, a common virus. (A different type of herpes virus causes genital and other herpes infections.) Although uncomfortable and occasionally painful, untreated cold sores usually dry up in 10 days to 3 weeks.
To relieve inflammation, try applying a compress soaked in refrigerated Burow’s solution (1:40 dilution) for 10 minutes five times daily. A severe outbreak should be seen by your doctor. Beware of touching the blister and then your eyes; this could cause an eye ulcer and infection. Also avoid touching anyone who has eczema; that person can easily get herpes all over his or her body.
After the sores disappear, the virus remains dormant and can be reactivated by illness (usually a cold), exposure to wind or sun, or stress or menstruation. Subsequent eruptions begin with tingling, burning, and itching, followed by the appearance of a cluster of blisters. If you often get cold sores, use a sun-blocking lip balm when outdoors.