Not every blister or sore on the lip is a cold sore. Cold sores are caused by the herpes simplex virus, usually herpes simplex virus type 1 (HSV 1). They often follow a recognisable pattern, starting with tingling, itching or burning before one or more small, fluid filled blisters appear on or around the lips. The blisters can then break, crust over and gradually heal.
If what you are seeing does not fit this pattern, other causes can include mucoceles, angular cheilitis, contact dermatitis, irritation from lip licking, burns, friction, mouth ulcers and hand, foot and mouth disease. These conditions are not all contagious and they do not need the same treatment.
This guide explains how cold sore symptoms differ from other causes of lip blisters and sores, what each condition may look and feel like, when to seek medical advice and which treatments may be appropriate. Knowing the possible causes can help you avoid using a treatment that is not suitable and seek the right advice if you need it.
Treatment suitability depends on an individual clinical assessment. This article is not a substitute for professional advice if you are unsure what is causing a blister or sore on your lip.
How to Tell a Cold Sore From Other Lip Blisters
Cold sores often begin with a tingling, itching or burning feeling around the lip. Within around 48 hours, one or more small, painful blisters may appear. The blisters can then burst and form a crust as they heal. Cold sores usually start to heal within 10 days and remain contagious from the first tingling or other early symptoms until they have completely healed.
If your blister does not follow this pattern, there may be another explanation. For example, a single clear or pale bump on the inside of the lower lip may be a mucocele, while a sore at the corner of the mouth may be angular cheilitis.
The appearance and location of a sore can provide clues, but they cannot always confirm the cause. If you are unsure, a pharmacist, GP or dentist can help assess it.
Mucocele: A Common Cause of a Single Clear Lip Blister
A mucocele, also called a mucous cyst, can cause a small, clear or bluish, dome shaped swelling. It usually appears on the inside of the lower lip.
Mucoceles can develop when a small salivary duct is damaged, sometimes after minor trauma such as biting the lip. This allows mucus or saliva to collect beneath the lining of the mouth.
Mucoceles are usually painless and are not contagious. Some clear up without treatment, although they can come back. If a mucocele is large, persistent or keeps returning, a dentist or other qualified healthcare professional can assess it and discuss whether treatment is needed.
Because a mucocele is not caused by a viral infection, antiviral cream such as aciclovir is not used to treat it.
Angular Cheilitis: Blisters at the Corners of the Mouth
If you have redness, cracking or soreness at one or both corners of your mouth, angular cheilitis is one possible cause.
It can be linked to moisture and saliva collecting at the corners of the mouth. Yeast or bacteria can also contribute in some cases. It may look red, cracked or crusted and can become uncomfortable when you open your mouth wide.
Angular cheilitis is different from a cold sore, so antiviral treatment is not usually the answer. Treatment depends on the cause. A pharmacist or GP can help work out whether irritation, an infection or another factor is contributing.
Can Licking Your Lips Cause Blisters?
Repeated lip licking can irritate the skin around the mouth and contribute to a type of irritant contact dermatitis sometimes called lip licker's dermatitis.
Constantly wetting the lips with saliva can make dryness and irritation worse. This may lead to cracking, peeling, redness and soreness around the lips.
It can be more noticeable in cold or dry weather, particularly if you already have dry lips. Try to avoid licking the affected area and protect the lips with a plain lip balm or petroleum jelly.
Antiviral treatment is not appropriate unless the symptoms are caused by a viral infection.
Contact Dermatitis and Allergic Reactions
An allergic or irritant reaction to something that has touched your lips can cause redness, swelling, itching and sometimes small blisters.
Possible triggers include some lip balms, lipsticks, toothpastes, skincare products and dental materials. Spicy or acidic foods can also irritate lips that are already sore.
Unlike a typical cold sore, contact dermatitis may affect a wider area and is often itchy. If you suspect a particular product is causing the problem, stop using it where appropriate and see whether the irritation improves. If symptoms continue or you are unsure about the cause, speak to a pharmacist, dentist or GP.
Burns and Friction Blisters
Hot food or drinks, strong sunlight and friction from braces, dental work or biting can all irritate or damage the lips and may cause blistering or soreness.
There may be an obvious trigger, such as drinking something very hot, spending time in strong sunlight or having new orthodontic work.
For a minor burn, cool running water can help. Avoid putting ice directly on the affected area, as this can damage the skin further. Seek medical advice if the burn is severe, covers a large area, becomes increasingly painful or does not seem to be healing.
Mouth Ulcers (Aphthous Ulcers)
Aphthous ulcers, commonly called mouth ulcers in the UK, usually develop on the soft lining inside the mouth, including the inside of the lips. They are usually shallow ulcers with a white or yellowish centre and a red border. They are not contagious and are not caused by the herpes virus.
Mouth ulcers can be linked to minor injury or irritation. Some people also find that stress, hormonal changes or nutritional deficiencies are associated with recurring ulcers.
Most mouth ulcers clear up within 1 or 2 weeks. If an ulcer lasts longer than 3 weeks, is different from your usual ulcers, becomes more painful or keeps coming back, speak to a GP or dentist.
Blisters in Children: Hand, Foot and Mouth Disease
In young children, mouth ulcers together with spots or blisters on the hands and feet may suggest hand, foot and mouth disease. It is a common viral infection that can also affect adults.
It can cause a sore throat, high temperature, mouth ulcers and a rash that may develop into blisters on the hands and feet. It usually gets better on its own within 7 to 10 days.
Treatment usually focuses on keeping the child comfortable and well hydrated. Cool drinks and soft foods can help if the mouth is sore. A pharmacist can also advise on suitable treatments for symptoms.
Speak to a GP or NHS 111 if a child seems very unwell, has a very high temperature or is drinking and urinating less than usual, as this can be a sign of dehydration.
Quick Comparison: Lip Blister Causes at a Glance
| Cause | Typical appearance | Contagious? |
|---|---|---|
| Cold sore | One or more small blisters on or around the lips, often preceded by tingling, itching or burning | Yes, from the early symptoms until fully healed |
| Mucocele | Single, clear or bluish swelling, usually on the inside of the lower lip | No |
| Angular cheilitis | Cracked, sore or inflamed skin at the corners of the mouth, sometimes with crusting | Not usually |
| Lip licker's dermatitis | Dry, irritated, cracked or sore skin around the lips | No |
| Contact dermatitis | Itchy, red or swollen area that may blister after contact with an irritant or allergen | No |
| Burn or friction blister | Blistering or soreness after a burn, rubbing or another form of irritation | No |
| Aphthous ulcer | Small, shallow ulcer with a pale centre and red border, usually inside the mouth | No |
| Hand, foot and mouth disease | Mouth ulcers with spots or blisters on the hands and feet, usually in children | Yes |
This table is a general guide only. The appearance of a lip blister cannot always confirm its cause, so speak to a healthcare professional if you are unsure.
When to See a Doctor or Pharmacist
Speak to a pharmacist or GP if a blister or sore is unusually large or painful, keeps coming back, is accompanied by a fever, or you feel generally unwell.
You should also seek advice if you are not sure what is causing the sore or if it is not healing as expected.
A mouth ulcer that lasts longer than 3 weeks should be checked by a GP or dentist. A persistent lump, sore or unusual change on the lip or inside the mouth should also be assessed. Most mouth problems are not serious, but persistent symptoms should not be ignored.
If It Does Turn Out to Be a Cold Sore
If your symptoms fit the typical cold sore pattern, such as tingling or burning followed by small blisters on or around the lips, treatment may be more useful when started early.
For example, NHS guidance recommends applying aciclovir cream as soon as you notice itching or tingling.
Cold sores are contagious from the first signs of tingling or other symptoms until they have completely healed. Washing your hands after touching the area and avoiding kissing or oral sex while you have a cold sore can help reduce the risk of spreading the virus.
EverydayMeds offers treatment options for cold sores, including aciclovir, subject to a clinical assessment by a UK registered prescriber.
