A fever blister is a small, fluid-filled sore that forms on or around the lips, caused by herpes simplex virus type 1 (HSV-1). You may also hear them called cold sores. They’re extremely common, and most people pick up the virus during childhood through casual contact like a kiss from a family member. Once you have HSV-1, it stays in your body permanently, lying dormant in nerve cells and occasionally reactivating to produce a new blister.
Fever Blisters vs. Canker Sores
These two get confused constantly, but they’re completely different conditions. Fever blisters appear on the outside of the mouth, typically along the outer edge of the lips. They look like a cluster of small, fluid-filled blisters and are caused by a viral infection. Canker sores show up inside the mouth and are usually a single round white or yellow sore with a red border. Canker sores aren’t caused by a virus at all. Their exact cause is unknown, though they can be triggered by mouth injuries, stress, smoking, or deficiencies in folic acid, iron, or vitamin B12.
The simplest way to tell them apart: if it’s outside your mouth on your lip, it’s likely a fever blister. If it’s inside your mouth on soft tissue, it’s likely a canker sore.
What a Fever Blister Looks and Feels Like
Fever blisters move through a predictable sequence over about a week. The first sign is a tingling, itching, burning, or numb sensation on your lip or the skin nearby. This warning phase, called the prodrome, is your body’s earliest signal that the virus is reactivating.
Within about 24 hours, small bumps appear on or around the lips. On average, three to five bumps form, though you might get more or fewer. These bumps quickly fill with clear fluid and become true blisters. The surrounding skin turns red, swells, and becomes painful.
By days two to three, the blisters rupture and ooze clear or slightly yellow fluid. This weeping phase is when the sore looks its worst and is at its most contagious. Over the next day or two, the oozing stops and a golden-brown crust forms. This scab gradually heals, and in most cases the skin returns to normal without scarring.
Why They Keep Coming Back
After your first infection, HSV-1 retreats into nerve cells near the base of your skull and goes dormant. It can stay quiet for months or years, then something tips the balance and the virus travels back along the nerve to the skin surface. The common thread in nearly every trigger is that your immune system gets distracted, weakened, or overwhelmed.
Illness is one of the most reliable triggers. Cold sores get their name because they so often appear alongside a common cold. Any infection can throw your immune system off balance, and fevers are especially effective at bringing on an outbreak. Stress plays a major role too. Short-term stress floods your body with cortisol and adrenaline, while long-term stress causes chronic inflammation that keeps your immune system busy and gives the virus an opening to reactivate.
Hormonal shifts during menstruation, pregnancy, puberty, or menopause can also trigger outbreaks, partly because certain hormones like progesterone slightly suppress immune function. Lack of sleep weakens your immune response in a similar way. Extreme temperatures, both hot and cold, stress your body enough to allow reactivation, and cold weather adds the extra problem of leaving your lips dry and cracked.
Physical trauma to the lips is another well-known trigger. Any kind of lip injury, even a bruise that doesn’t break the skin, can set off an outbreak. Cosmetic procedures like filler injections, permanent makeup, and lip flips carry the same risk. People who are immunosuppressed due to cancer treatment, organ transplant medications, or conditions like lupus tend to experience more frequent outbreaks because their immune systems are either deliberately dialed down or chronically overworked.
How the Virus Spreads
HSV-1 spreads through direct contact with the virus, most commonly through saliva, skin-to-skin contact around the mouth, or contact with an active sore. You’re most contagious when blisters are present, especially during the weeping phase when fluid is leaking from open sores. But the virus can also shed from skin that looks completely normal, which means transmission is possible even when you have no visible symptoms and feel fine.
This is why the virus is so widespread. Many people who carry HSV-1 don’t know they have it, because they either never develop visible sores or have outbreaks so mild they go unnoticed. Sharing utensils, lip balm, or towels with someone during an active outbreak increases the risk. Kissing is the most common route. Less commonly, HSV-1 can spread to the genital area through oral sex.
Treatment and Timing
Most fever blisters heal on their own within seven to ten days. Antiviral medications can shorten that timeline and reduce the severity, but only if you start them early. The critical window is the prodrome phase, that initial tingling or burning before blisters appear. Starting antiviral treatment at this point gives the best results. Once blisters have fully formed, antivirals are less effective though they can still help somewhat.
Over-the-counter topical creams can ease pain and may modestly speed healing. Keeping the area clean and avoiding picking at the scab helps prevent secondary bacterial infection and scarring. Ice or a cool compress can relieve some of the swelling and discomfort during the blister phase.
If you get frequent outbreaks, a doctor can prescribe daily antiviral medication taken long-term to suppress the virus and reduce how often sores appear.
Prevention: What Works and What Doesn’t
Avoiding your personal triggers is the most practical prevention strategy. If stress, poor sleep, or illness tend to precede your outbreaks, addressing those factors can reduce how often the virus reactivates. Protecting your lips from sun damage seems logical, and sunscreen does block cold sores triggered by artificial UV light in lab settings. But a Cochrane review of the evidence found that sunscreen did not prevent cold sores triggered by natural sunlight, making its real-world benefit uncertain.
Lysine, an amino acid widely sold as a cold sore supplement, is another popular remedy. Despite its reputation, the same Cochrane review found no evidence that lysine prevents fever blisters. If you want proven prevention for frequent outbreaks, daily prescription antivirals remain the most reliable option.
Complications Worth Knowing About
For most people, fever blisters are a nuisance, not a danger. But the virus can sometimes spread to other parts of the body, and two locations deserve attention.
The eyes are vulnerable because it’s easy to transfer the virus from a lip sore to your eye by touching one and then the other. Eye herpes can cause redness, pain, light sensitivity, watery eyes, swelling of the eyelids, and a feeling like something is stuck in your eye. In more serious cases, it can inflame the cornea. If you develop eye symptoms during or shortly after a fever blister outbreak, get it checked promptly, because untreated eye herpes can affect your vision.
The simplest way to prevent this is to wash your hands thoroughly after touching a fever blister and to avoid rubbing your eyes during an outbreak. The same hand-hygiene rule applies to prevent spreading the virus to your fingers or to other people.