Cold Sores: Causes, How Long They Last, Which Medicines Really Work, and How to Tell Them Apart from Canker Sores and Angular Cheilitis, Explained by a Dermatologist
A cold sore is an infection with herpes simplex virus type 1 (HSV-1) on or around the lips. It causes small clusters of clear blisters on a red base, with burning, which then break open into sores that crust over. The sore usually heals on its own within 7-10 days, but the virus does not go away. It retreats into a nerve ganglion and can keep coming back in the same spot.
What most people get wrong is when they start treatment. Antiviral medicine works best when it is started within the first 48-72 hours, especially while you can still feel tingling or stinging before the blisters appear. If you wait until the blisters have broken into sores before getting medicine, the benefit drops considerably.
This article explains how the disease works, how to tell it apart from other conditions that look similar, which medicines have real evidence behind them, what does not help, and the warning signs that mean you should see a doctor promptly.
What is a cold sore?
A cold sore (Herpes labialis), which many people call a "fever blister" or "cold sore", is an HSV infection of the lips, the corners of the mouth and the skin around the mouth. For an overview of herpes infections in general, see Herpes: causes and treatment.
The features that are characteristic of a cold sore are:
Several small clear blisters grouped together, not a single blister. Each blister is usually smaller than 3 millimetres.
A red base under the cluster, with a fairly clear border.
Warning symptoms beforehand, such as tingling, stinging, tightness or a sharp pain at the same spot, about half a day to one day before the blisters appear.
It recurs at the same spot or very close to it every time, because the virus lives in the same nerve ganglion.
It is most common on the outer edge of the lip, at the border between the lip and the skin.
In epidemiological terms, HSV-1 infection is very common worldwide. Many people are infected in childhood without obvious symptoms and only develop cold sores later in life. Having a cold sore therefore does not mean poor hygiene, and it does not always mean the infection came from sexual contact.
What causes cold sores?
The cause is a virus from the herpes family. Two types are involved in sores around the mouth:
HSV-1 causes most cold sores. It spreads through saliva and direct contact, such as kissing, or sharing drinking glasses, spoons or lip balm.
HSV-2 is the main cause of genital herpes, but it can also appear on the mouth through oral sex, and likewise HSV-1 can settle in the genital area.
What makes herpes different from an ordinary skin infection is that after the first infection, the virus travels back up the nerve and settles quietly in the trigeminal ganglion, in a state called latency. The immune system cannot clear it from there, and antiviral drugs cannot kill the virus while it is latent.
This is why we have to be straightforward: there is currently no treatment that removes the herpes virus from the body permanently. What treatment can do is make each episode heal faster, make symptoms milder and reduce how often it comes back.
Why does it keep coming back in the same place?
When a trigger occurs, the hidden virus starts multiplying again and travels down the same nerve, so it always appears on the same area of skin.
One-minute video summary by Dr. Patara Voravutinon, Dermatologist: Why do cold sores on the lip appear as clear, stinging, itchy blisters that return to the same spot every time? The answer is that the herpes virus (HSV-1) never leaves the body. It hides in the nerve ganglion that supplies that area of skin. When you are short on sleep, stressed, ill or exposed to strong sun, the virus travels back out along the same nerve, so it appears in the same place, and stinging or itching usually comes 6-48 hours before you see the blisters.
Common triggers
Strong sunlight and UV radiation are among the clearest triggers. Many people notice a cold sore after a beach trip or a long time in the sun.
Fever and illness, which is where the English name fever blister comes from.
Stress and lack of sleep.
Menstruation, because of hormonal changes.
Injury to the lips, such as very dry, cracked lips, procedures around the mouth, or dental work.
Weakened immunity, for example from immunosuppressive drugs, chemotherapy or an underlying condition that suppresses the immune system.
Keeping a note of what you were exposed to before each flare-up is genuinely useful, because it lets you plan prevention in advance. For example, if you always flare after being in the sun, using a lip balm with sunscreen regularly can reduce the chance of an episode.
Symptoms of a cold sore: 5 stages
Knowing the stage matters in practice, because medicine works very differently at each stage.
Prodrome, about 6-24 hours. No blisters are visible yet, but you feel tingling, stinging, tightness or a sharp pain at the usual spot. This is when starting medicine gives the greatest benefit.
Vesicle (blister) stage, days 1-2. Small clear blisters cluster on a red base. The amount of virus is high at this stage and it is the most contagious.
Ulcer (open sore) stage, days 2-4. The blisters break and become shallow red sores. This is when it hurts the most.
Crust stage, days 4-8. The sore dries into a yellow-brown scab. The scab can crack and bleed a little when you move your mouth.
Healing stage, days 8-12. The scab falls off. The skin underneath may stay red or darker for a while, then fade.
The first infection is usually much more severe than later recurrences, especially in young children, who may have sores inside the mouth and swollen, red gums, fever and be unable to eat. This is called herpetic gingivostomatitis. These children should see a doctor because of the risk of dehydration.
How many days does a cold sore take to heal?
Typical recurrence: the sore usually heals on its own within 7-10 days without scarring.
First infection: it can take longer, about 10-14 days, with symptoms affecting the whole body.
If an oral antiviral is started at the prodrome stage, the sore usually lasts about 1-2 days less and the pain is reduced.
This figure matters because it lets you set realistic expectations. Antiviral medicine does not make the sore heal in one day. What it can do is shorten the course and make symptoms milder. If the sore has not improved at all after 10-14 days, or gets worse, see a doctor to review the diagnosis and check for a secondary bacterial infection.
Telling a cold sore apart from 5 look-alike conditions
This is the most common reason for wrong treatment, because each condition needs a completely different type of medicine.
1. Canker sore (Aphthous ulcer)
Location: on the lining of the mouth, such as the inside of the cheek, under the tongue or the inner lip. It does not appear on the outer skin.
Appearance: a round or oval single sore with a white-yellow floor and a clearly red edge, with no blister stage beforehand.
Treatment: antivirals do not help. A topical anaesthetic or a topical steroid for the mouth is used as your doctor prescribes.
2. Angular cheilitis
Location: mainly at both corners of the mouth.
Appearance: red, cracked, fissured, peeling skin with no cluster of blisters. It is often chronic and lasts several weeks.
Causes: Candida yeast, bacteria, saliva pooling at the corners of the mouth, poorly fitting dentures, or iron and vitamin B deficiency.
Treatment: a topical antifungal or topical antibiotic depending on the cause, together with correcting the contributing factors. See also Angular cheilitis.
3. Impetigo
Appearance: pustules or sores with a honey-coloured, golden-yellow crust, which is characteristic.
Spread: it keeps spreading over a wider area and moves to other parts of the body through scratching.
Cause: Staphylococcus or Streptococcus bacteria.
Treatment: antibiotics, not antivirals. See also Impetigo.
4. Contact cheilitis
Appearance: swollen, red, itchy, flaky lips. There may be small blisters, but they are spread symmetrically across the lips, not clustered in a single patch.
Clue: it is linked to lipstick, lip balm, toothpaste or something newly started.
Treatment: stop the irritant and use an anti-inflammatory topical as your doctor prescribes.
5. Hand, foot and mouth disease (HFMD)
Appearance: several sores in the mouth together with blisters on the palms, soles and around the buttocks, and fever.
Who gets it: mainly young children, often in outbreaks at school.
Treatment: supportive care for symptoms. See also Hand, foot and mouth disease.
The quickest summary clue: if you have small clear blisters grouped on a red base at the outer edge of the lip, with tingling or stinging beforehand and a history of the same spot being affected before, the likelihood of a cold sore is very high.
How does a doctor diagnose a cold sore?
In practice, a cold sore with a clear appearance and history can be diagnosed by physical examination without further tests.
Testing is considered when the picture is not typical, the patient has weakened immunity, or complications are suspected. The methods used include:
PCR to detect the virus's genetic material from fluid in a blister or the base of a sore. This is currently the most sensitive and specific method.
Viral culture is accurate but takes longer, and is less sensitive once the sore has begun to crust.
Tzanck smear is a stain of cells scraped from the base of the sore. It is quick but cannot distinguish herpes from chickenpox or shingles, and is less sensitive than PCR.
Blood tests for antibodies can only tell whether you have been infected before. They cannot be used to diagnose the sore in front of you, because many people have positive antibodies without ever having had symptoms.
Samples give the best results when taken at the blister stage or when the sore has only just opened, not once it has crusted.
Cold sore medicines: which ones really work?
The main drugs are nucleoside analogue antivirals: aciclovir, valaciclovir and famciclovir. They work by blocking the virus from multiplying while it is actively dividing, so they must be given while the virus is active, which is the early part of a flare-up.
Episodic therapy
This is used when flare-ups are infrequent, starting the medicine as early as possible from the first warning symptoms. International guidelines describe commonly used regimens such as a short, high-dose, one-day course of valaciclovir or a single oral dose of famciclovir, which are more convenient than taking aciclovir several times a day for several days.
Suppressive therapy
This is considered for people who get frequent flare-ups, for example about 6 or more times a year, who have severe symptoms every time, whose work or quality of life is affected, or who have weakened immunity. It involves taking a low-dose antiviral every day for a period of time, then reassessing.
An important point: oral antivirals are prescription medicines whose dose is set by a doctor or pharmacist. The dose has to be adjusted for kidney function, age and pregnancy, so you should not set the dose yourself or use someone else's leftover medicine.
Topical cold sore creams: why they work less well than many people think
Topical antiviral creams available at pharmacies, such as aciclovir cream, have limitations you should understand.
Limited benefit. Studies show they shorten the sore by only about half a day to one day, clearly less than oral medicine.
They must be applied very often. Generally about 5 times a day for several days, which most people do not manage in practice.
The drug penetrates scabs poorly. Once the blister stage has passed, the benefit falls further.
Practical summary: topical creams suit mild symptoms, extra comfort, or use while waiting to see a doctor. If you get them often, have severe symptoms or a wide sore, oral medicine is the better-supported option.
For general care of the sore, clean it gently, let it dry, apply a thin layer of petroleum jelly or lip balm to reduce cracking of the scab, and use a brief cold compress to ease pain and swelling.
What does not help, and common myths
Squeezing or picking the blisters does not make it heal faster. It raises the chance of a secondary bacterial infection, makes the sore wider and risks scarring, and can spread the virus to your fingers or eyes.
Putting toothpaste on it. There is no evidence that it kills the virus, and the ingredients often irritate the skin around the mouth and make it more inflamed.
Oral or topical antibiotics do not work on viruses. They are used only when there is a secondary bacterial infection that a doctor has assessed.
Topical steroid used alone on a cold sore can make the sore spread more because it suppresses local immunity. Do not use leftover rash cream from home on it.
Lysine supplements or herbal remedies. The evidence is not strong and study results conflict, so they should not replace an antiviral that is indicated.
The belief that it is not contagious once it has crusted. The virus can still be found until the sore has healed completely, so keep taking care until the scab has fallen off and the skin has closed.
How cold sores spread, and how to avoid passing them on
It spreads through direct contact with fluid from the blisters, the sore or the saliva of an infected person, and it can spread even at times when no sore is visible, which is called asymptomatic shedding.
While you have a sore, you should do the following:
Avoid kissing, especially babies, young children, pregnant women and people with weakened immunity.
Avoid oral sex, because the virus can pass to your partner's genitals.
Do not share drinking glasses, cutlery, towels, lip balm or razors with others.
Wash your hands every time after touching the sore, and never rub your eyes, because if the virus reaches the eye it can cause corneal inflammation.
If you wear contact lenses, be especially careful when handling them while you have a sore.
Warning signs: when to see a doctor
Most cold sores are not dangerous, but there are situations that need prompt assessment.
Eye symptoms such as a red eye, eye pain, blurred vision, watering, sensitivity to light, or blisters near the eyelid or the tip of the nose. This may be an infection of the cornea, which needs prompt care from an eye doctor because it threatens vision.
People with eczema who develop blisters or punched-out sores spreading quickly over a wide area, together with fever. This may be eczema herpeticum, which is a skin emergency. See also Eczema.
People with weakened immunity, for example on immunosuppressive drugs or chemotherapy, after an organ transplant, or with HIV infection. The sore may spread widely, become deep and heal unusually slowly.
Babies younger than 3 months with blisters or sores like this need to see a doctor immediately, because infection in a newborn can be severe.
A sore that has not improved at all after 10-14 days, or gets worse, with pus, increasing pain or a high fever.
Flare-ups so frequent that they affect daily life. You should consult a doctor about suppressive therapy.
An unusual chronic sore on the lip that does not heal for a month should be examined to rule out other conditions, including precancerous lesions of the lip.
Cold sores in specific groups
Young children. The first infection is usually more severe than in adults. There may be swollen red gums, sores spread through the mouth, fever, and being unable to eat or drink. The most important thing is to watch for dehydration, and the child should see a doctor for assessment.
Pregnant women. A cold sore during pregnancy generally does not directly harm the baby in the womb, but you need to be careful about passing the virus to the baby after birth. Avoid kissing the baby while you have a sore, wash your hands before touching the baby every time, and consult a doctor before using any medicine.
People with weakened immunity. They should see a doctor at every flare-up, because they may need a higher dose, a longer course, or intravenous medicine in severe cases.
People about to have a facial procedure. For example certain skin lasers or chemical peels. Anyone with frequent cold sores should tell the doctor beforehand, because the doctor may consider giving a preventive antiviral before the procedure.
How to reduce the chance of recurrence
Use a lip balm with sunscreen regularly, especially if your flare-ups follow sun exposure, and reapply when you are outdoors for a long time.
Keep your lips from getting dry and cracked, because injury to the skin is a trigger.
Get enough sleep and manage stress, which are common triggers.
Do not share personal items with others.
Keep a record of when flare-ups happen and what came before to find your own personal triggers.
Have medicine ready in advance as your doctor advises if you flare regularly, so you can start it in time during the warning stage. This is the one factor you can control that has the clearest effect.
Frequently asked questions about cold sores
How many days does a cold sore take to heal?
A typical recurrence usually heals on its own within 7-10 days, while a first infection can take 10-14 days. Starting an oral antiviral while you still have tingling or stinging, before the blisters appear, can shorten the course by about 1-2 days and make symptoms milder.
Can a cold sore be cured completely?
Each sore heals, but the virus stays in the nerve ganglion permanently. There is currently no way to remove the virus from the body. What can be done is to make each episode heal faster, make symptoms milder, and reduce how often it recurs with suppressive therapy in people who get it often.
How can I tell whether it is a cold sore or a canker sore?
A cold sore is usually a group of small clear blisters on a red base on the outer lip or the skin around the mouth, with tingling or stinging beforehand, and it recurs in the same spot. A canker sore is a single sore with a white-yellow floor and a red edge inside the lining of the mouth, with no blister stage beforehand. If you are not sure, have a doctor examine it, because the medicines are completely different.
Is a cold sore cream from the pharmacy enough?
Creams help a little, shortening the sore by about half a day to one day, and they must be applied many times a day, so they suit mild symptoms. If you get cold sores often, have severe symptoms, a wide sore or weakened immunity, an oral medicine prescribed by a doctor is clearly better supported by evidence.
How do cold sores spread?
They spread through direct contact with fluid from the blisters, the sore or saliva, for example by kissing, sharing drinking glasses, spoons or lip balm, and through oral sex, which can pass the virus to a partner's genitals. The virus can also spread at some times when no sore is visible. While you have a sore, avoid kissing and do not share personal items.
What should I do if I get cold sores very often?
If you flare about 6 or more times a year, have severe symptoms every time, or your quality of life is affected, consult a doctor about suppressive therapy, which is a continuous low-dose antiviral, together with finding and avoiding your personal triggers such as sunlight or lack of sleep.
Consult a dermatologist
If you have a sore on the mouth and are not sure whether it is a cold sore or something else, you get cold sores so often that they disrupt daily life, or you have any of the warning signs above, getting the right diagnosis early helps you choose the right type of medicine and reduces complications.
Consult a dermatologist at Siam Dermatology, call +66 61 448 7000 or message us on LINE @siamderm.
Written by Dr. Patara Voravutinon, Dermatologist · Last reviewed August 27, 2026
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