A cold sore is a small, often painful blister or cluster of blisters on or around the lips caused by the herpes simplex virus, which stays in the body for life and can reactivate.
That first tingle is the giveaway. It usually shows up before you can see anything, and by the time a blister appears, the virus has already settled in for another round. Cold sores come from the herpes simplex virus, most often type 1 (HSV-1), and once you have it, it never fully leaves. The sections below cover what triggers an outbreak, how long one lasts, and when it’s worth calling a doctor.
What Causes a Cold Sore to Form?
A cold sore forms when the herpes simplex virus reactivates inside nerve cells and travels to the skin near your lips. The first infection is the primary cause, but every outbreak after that is a reactivation of virus that was already there.
HSV-1 is the usual culprit; HSV-2, the type more often linked to genital herpes, can also cause cold sores, just less commonly. After the initial infection, the virus lies dormant in nerve cells and wakes up when something gives it an opening.
Common reactivation triggers include:
- Illness or fever (this is where the old name “fever blister” comes from)
- Stress and fatigue
- Hormonal changes, including menstruation
- Sun and wind exposure
- Skin injury or trauma to the area
- Shifts in immune-system function
- Dental work or cosmetic and laser procedures
Not everyone reacts to the same trigger. Some people go years between outbreaks with no obvious reason, while others can almost predict one after a stressful week or a long day in the sun.
How Does a Cold Sore Spread?
Cold sores spread through close contact with the virus, including during an active outbreak. Kissing is the most common route, but oral sex can pass HSV-1 or HSV-2 between mouth and genitals in either direction.
Shared objects matter too. Utensils, razors, towels, and similar items can carry the virus from one person to another. The fluid inside a blister is contagious, so touching a sore and then touching another part of your body — or someone else — is a real risk.
Most people pick up HSV-1 in childhood through everyday contact, often before anyone realizes it’s there. Once you have the virus, your body builds antibodies, but those don’t clear it. They just make later outbreaks milder and less frequent for many people.
What Does a Cold Sore Look and Feel Like?
A cold sore usually announces itself before you see it. Tingling, burning, itching, or tenderness around the lips is the classic early sign, sometimes a day or two before any blister shows up.
Then the blister appears, often in a small cluster. It breaks, crusts over, and heals. Without treatment, a cold sore typically runs its course in about 2 to 3 weeks, and most heal without scarring.
| Stage | What You Notice | Typical Timing |
|---|---|---|
| Tingling | Itching, burning, or tenderness near the lips | First sign, before any blister |
| Blister | Small, fluid-filled bump or cluster | Appears shortly after tingling |
| Weeping | Blister breaks, oozes, forms a crust | Early healing phase |
| Crusting | Yellow or brown crust forms over the site | Mid-healing |
| Healing | Crust flakes off, skin returns to normal | Full course: about 2 to 3 weeks |
| Dormancy | No symptoms; virus stays in nerve cells | Until the next trigger |
Catching it early — right at the tingling stage — is when over-the-counter options tend to do the most good. That’s the same window where the products in our roundup of tested cold sore treatment products are designed to be used. Your clinician can confirm a diagnosis, which is usually made by looking at the sore rather than by testing.
When Should You See a Doctor About a Cold Sore?
Medical evaluation is warranted if cold sores are severe, keep coming back, or come with eye symptoms — eye involvement needs prompt attention. Diagnosis is usually clinical, meaning a health care provider identifies it by examining the sore.
Also worth a call: sores that don’t heal within the expected 2-to-3-week window, outbreaks that interfere with eating or drinking, or any sore near the eye or on broken skin. If you have a weakened immune system, the threshold for getting checked is lower.
Cold sores are contagious, including during an active outbreak and through direct contact with sores or infected fluid. That’s the reason lip balm, straws, and towels shouldn’t be shared while a sore is healing. NIH’s guidance on fever blisters and canker sores covers how the two often get confused, even though they have different causes.
FAQs
Can a cold sore come back in the same spot?
Often, yes. Because the virus lives in nerve cells and travels the same pathway to the skin, outbreaks frequently return to nearly the same location on or around the lips. That said, the exact spot can shift slightly, and some people get them in different places over time.
Are cold sores always caused by HSV-1?
Most cases are HSV-1, but HSV-2 can cause them too, just less often. The two types can also swap regions through oral sex, so a mouth sore isn’t automatically HSV-1. A health care provider can clarify which type is involved if it matters to you.
How long is a cold sore contagious?
A cold sore is most contagious from the first tingle until the blister has fully crusted and healed — roughly 2 to 3 weeks. Some sources note the virus can still shed in small amounts between outbreaks, though the risk is far lower than during an active sore.
References & Sources
- Mayo Clinic. “Cold sore — Symptoms and causes.” Supports causes, triggers, and typical healing timeline.
- NIH National Institute of Dental and Craniofacial Research. “Fever Blisters and Canker Sores.” Supports the distinction between fever blisters and canker sores.
- NHS. “Cold Sores.” Supports contagiousness, transmission routes, and duration.