A fever blister is a small, fluid-filled blister that forms on or around the lips, caused by an infection with herpes simplex virus type 1 (HSV-1). You may also hear them called cold sores. They’re extremely common, and most people who carry the virus picked it up during childhood through casual contact like a kiss from a family member. After the first infection, the virus stays in your body permanently, hiding in nerve cells and occasionally reactivating to produce a new blister.
What Causes Fever Blisters
HSV-1 is the virus behind nearly all fever blisters. It spreads through direct contact with sores, saliva, or skin surfaces in and around the mouth. You can also catch it from someone who has no visible sore at the time, though the risk is highest when an active blister is present. Less commonly, HSV-1 can spread to the genital area through oral-genital contact.
Once you’re infected, the virus travels along nerves and settles into nerve cells near the base of the skull, where it can remain dormant for months or years. Researchers at the University of Virginia School of Medicine found that when nerve cells harboring the virus experience a burst of overactivity (triggered by things like stress or illness), the virus senses that change and seizes the opportunity to reactivate. That reactivation is what produces a new fever blister.
Common Triggers for Outbreaks
Not everyone who carries HSV-1 gets frequent outbreaks. Some people have one and never see another; others get several a year. The triggers that wake the virus up tend to fall into a few categories:
- Physical stress on the body: fever, illness, surgery, or fatigue
- Sun exposure: UV light on the lips is one of the most reliable triggers
- Emotional stress: periods of anxiety, sleep loss, or high pressure
- Hormonal shifts: menstruation is a common trigger for some women
- Skin trauma: dental procedures, windburn, or chapped lips
If you notice a pattern in your outbreaks, avoiding that specific trigger can reduce how often they return. Wearing SPF lip balm year-round, for example, helps people whose blisters are sun-related.
The Five Stages of a Fever Blister
Fever blisters follow a predictable progression from start to finish, typically lasting 7 to 10 days.
Stage 1: Tingling
Before anything is visible, you’ll feel a tingling, burning, or itching sensation on or near your lip. This is the earliest warning sign, and it’s the best window to start treatment if you have antiviral medication on hand.
Stage 2: Blistering
About a day or two after the tingling starts, one or more small blisters filled with clear fluid appear on the skin’s surface. The surrounding skin will be red and swollen.
Stage 3: Weeping
Within a few days, the blisters break open into shallow, red sores that ooze fluid. This is the most contagious stage. The open sore can be painful, especially when eating, drinking, or talking.
Stage 4: Crusting
The open sore dries out and forms a yellowish or brownish crust. The scab may crack and bleed, which can be uncomfortable but is part of normal healing.
Stage 5: Healing
The scab gradually flakes away as new skin forms underneath. Emollients containing zinc oxide or aloe vera can keep the scab soft and reduce irritation during this stage. Once the scab is completely gone, the skin may appear slightly pink for a few more days, but the blister is no longer contagious once it’s fully healed.
Fever Blisters vs. Canker Sores
People often confuse fever blisters with canker sores, but they’re completely different conditions. The quickest way to tell them apart is location: fever blisters appear on the outside of the mouth, around the lips, while canker sores develop inside the mouth on soft tissue like the inner cheek, gums, or tongue.
They also look different. A fever blister is a cluster of small, fluid-filled blisters, while a canker sore is typically a single round sore that’s white or yellow with a red border. The cause is different too. Fever blisters come from HSV-1 and are contagious. Canker sores are not caused by a virus, are not contagious, and their exact cause is unknown, though they may be triggered by mouth injuries, stress, or nutritional deficiencies in iron, vitamin B12, or folic acid.
How Fever Blisters Spread
The virus passes through direct contact with an active sore, saliva, or infected skin. Kissing is the most obvious route, but sharing utensils, cups, towels, lip balm, or razors can also transmit it. Both HSV-1 and HSV-2 are most contagious when sores are present, but the virus can also spread when no symptoms are visible. The greatest risk, however, is during an active outbreak.
You can also spread the virus to other parts of your own body. Touching an open blister and then rubbing your eye, for instance, can introduce the virus to the cornea. This is why hand hygiene matters during an outbreak.
Treatment Options
There’s no cure for HSV-1, but antiviral medications can shorten outbreaks and reduce their severity. Prescription antivirals work best when taken at the first sign of tingling, before a blister fully forms. One common prescription regimen involves taking a high dose twice in a single day, 12 hours apart, which can sometimes prevent the blister from developing at all or significantly cut healing time.
Over-the-counter creams containing the antiviral docosanol can also help if applied early, though they tend to be less effective than prescription options. For pain relief, topical numbing agents or simple ice applied to the area can reduce discomfort. People who get frequent outbreaks (six or more per year) may benefit from daily suppressive antiviral therapy, which reduces both the number of outbreaks and the risk of transmitting the virus to others.
Preventing Spread and Reinfection
During an active outbreak, avoid kissing or sharing anything that touches your mouth: silverware, cups, towels, washcloths, and cosmetics. Once the blister has fully healed, throw away your toothbrush, lip balm, and any makeup that may have contacted the sore. These items can harbor the virus and reinfect you, triggering a new blister in the same spot.
Wash towels, washcloths, pillowcases, and sheets that you used while the sore was active. And resist the urge to pick at or peel the scab. Beyond slowing healing, touching the blister and then touching other parts of your face increases the risk of spreading the virus to a new area.
Rare but Serious Complications
For most people, fever blisters are a nuisance, not a danger. But in certain situations, HSV-1 can cause real harm. The most concerning complication is when the virus reaches the eye, causing a condition called herpes keratitis. This involves inflammation of the cornea that can lead to reduced sensation in the eye, scarring, and in severe cases, vision loss. If you develop eye pain, redness, or blurry vision during or shortly after a fever blister outbreak, that warrants urgent medical attention.
People with weakened immune systems, whether from chemotherapy, organ transplant medications, or conditions like HIV, are at higher risk for severe or prolonged outbreaks that may require longer courses of antiviral treatment. Newborns are also vulnerable if exposed to active sores, so avoiding contact between a fever blister and an infant is important.