What Is It?
A cold sore is a small, painful blister or cluster of blisters that usually appears on or around the lips. It is most commonly caused by the herpes simplex virus type 1, often shortened to HSV-1.
HSV-1 is the most common cause, although HSV-2 can also occasionally cause sores around the mouth. Cold sores are usually not dangerous in healthy adults, but they are contagious and can be uncomfortable, visible, and likely to recur.
A typical cold sore may start with tingling, itching, burning, or tightness before a blister appears. Cold sores can be contagious from the tingling stage until the skin has fully healed, especially when blisters, fluid, or crusting are present.
Cold sores usually develop over 1 to 2 days, then blister, break, crust, and heal within about 7 to 10 days. They are usually considered likely when a person has repeated lip blisters in a similar area, especially with tingling before the blister forms.
A first episode may be more painful and may come with fever, swollen glands, or several mouth sores. Other conditions can look similar, including mouth ulcers, angular cheilitis, impetigo, allergic lip irritation, cracked lips, acne near the mouth, hand, foot and mouth disease, or other viral skin infections.
Why Cold Sores Are So Common
Cold sores are common because HSV can stay inactive in the body after a first infection and reactivate later. Many people carry the virus without always having symptoms.
People often search for cold sores because they may appear suddenly, feel sore or embarrassing, and raise concerns about spreading the virus. Some people notice cold sores during stress, illness, tiredness, sun exposure, fever, hormonal changes, or after lip trauma.
What Causes It?
Cold sores are caused by herpes simplex virus. After the first infection, the virus can remain dormant in nerve tissue and reactivate from time to time.
Common Causes and Triggers
Previous HSV infection
Once HSV is in the body, it can reactivate and cause recurrent cold sores.
Stress, tiredness, or illness
Being run down, unwell, or sleep-deprived may trigger outbreaks in some people.
Sun exposure or lip irritation
Strong sunlight, dry cracked lips, dental work, or lip injury may trigger cold sores.
Fever or hormonal changes
Fever, menstruation, or other body changes can sometimes trigger recurrence.
Close contact during active sores
Cold sores can spread through kissing, oral contact, shared lip products, cups, cutlery, towels, or touching the sore then touching another area.
Cold sores are different from mouth ulcers. Cold sores usually occur on or around the lips and are contagious. Mouth ulcers usually occur inside the mouth and are not usually contagious.
What Should You Do?
If you suspect a cold sore, first avoid touching, picking, or squeezing it. Wash hands after contact and avoid kissing or sharing items that touch the mouth while symptoms are active.
Avoid touching the sore and then touching the eyes, as herpes infection near the eye needs urgent assessment. People with widespread eczema or broken skin should seek advice early because herpes infection can spread more seriously on damaged skin.
What to Observe First
Pay attention to:
- Whether tingling, itching, or burning happened before the blister
- Whether the blister is on or around the lip
- Whether similar sores have happened before
- Whether there is spreading redness, pus, fever, or severe pain
- Whether the sore is near the eye
- Whether eating, drinking, or swallowing is affected
- Whether the person is a baby, pregnant, has eczema, or has weakened immunity
How to Tell If It Is Mild, Moderate, or Concerning
Mild cold sores may involve one small cluster of lip blisters, mild soreness, and no fever or spreading redness.
Moderate cold sores may be more painful, recurrent, larger, or interfere with eating, drinking, school, work, or confidence.
Concerning symptoms include sores near the eye, widespread blisters, severe pain, fever, pus, dehydration, or cold sores in babies or people with weakened immunity.
How Is It Usually Managed?
Cold sores are usually managed by reducing spread, protecting the area, and monitoring healing. General steps include hand hygiene, avoiding close contact during active sores, not sharing lip products, and avoiding picking the crust.
A pharmacist can help assess whether the sore sounds like a cold sore or whether another condition may be possible. Pharmacist advice is especially useful if symptoms are early, recurrent, painful, or difficult to distinguish from mouth ulcers, impetigo, or allergic irritation.
Ask a Pharmacist If Unsure
Ask a pharmacist if the sore is mild but uncomfortable, keeps recurring, or you are unsure whether it is a cold sore.
Seek medical advice earlier for babies under 6 months old, children with severe symptoms, pregnant women, people with eczema, weakened immunity, cancer treatment, or sores near the eye.
When to See a Doctor
See a doctor if cold sore symptoms are linked with:
- Sores near or in the eye
- Baby under 6 months old
- Severe pain, spreading redness, pus, or swelling
- Fever, feeling very unwell, or widespread blisters
- Difficulty drinking, dehydration, or reduced urination
- Weakened immunity, cancer treatment, organ transplant, or HIV
- Severe eczema or widespread skin involvement
- First episode that is severe or difficult to recognise
- Cold sores that do not heal after 10 to 14 days
- Frequent or unusually severe recurrences
Quick Summary
- A cold sore is a contagious lip blister usually caused by HSV-1.
- HSV-2 can occasionally cause sores around the mouth.
- It often starts with tingling, itching, burning, or tightness.
- Cold sores can spread from the tingling stage until fully healed.
- Seek medical advice for eye involvement, babies under 6 months, severe symptoms, dehydration, weakened immunity, severe eczema, or non-healing sores.
FAQ
What is a cold sore?
A cold sore is a small blister or cluster of blisters, usually on or around the lips, commonly caused by herpes simplex virus.
Is a cold sore serious?
Most cold sores are not serious in healthy adults, but they need medical advice if severe, near the eye, widespread, or affecting babies or people with weakened immunity.
When is a cold sore most contagious?
A cold sore can spread from the tingling stage until the sore has completely healed, especially when blisters, fluid, or crusting are present.
Can I spread a cold sore to my eye?
Yes, it is possible. Avoid touching the sore and then touching the eye. Eye pain, redness, light sensitivity, or sores near the eye need urgent medical advice.
How long does a cold sore last?
A typical cold sore often heals within 7 to 10 days. If it does not heal after 10 to 14 days, it should be assessed.
Can a cold sore go away on its own?
Yes. Many cold sores heal on their own, but they can recur because the virus remains inactive in the body.
Is a cold sore the same as a mouth ulcer?
No. Cold sores usually occur on or around the lips and are contagious. Mouth ulcers usually occur inside the mouth and are not usually contagious.
When should I see a doctor for a cold sore?
See a doctor if the sore is near the eye, occurs in a baby under 6 months old, is severe or widespread, causes dehydration, does not heal after 10 to 14 days, or occurs in someone with weakened immunity.