Blister on Lip: Causes, Symptoms, and When to See a Doctor

A lip blister is a small, raised, fluid-filled lesion that may form from irritation, trauma, infection, allergy, or sun exposure. Many people assume lip blisters are always cold sores, but several non-contagious conditions can cause similar bumps.
Cold sores are caused by herpes simplex virus type 1 (HSV-1) and are contagious [CDC], while most other lip blisters are not.
Seek medical care if a blister is unusually painful, lasts more than two weeks, recurs often, or is accompanied by fever or spreading redness.

What Is a Lip Blister?

A lip blister is a localized pocket of fluid that develops on the outer lip due to inflammation, irritation, or damage. Blisters may appear clear, yellowish, or reddish depending on the cause and healing stage.

Blisters on the outside of the lip can result from trauma, infection, sunburn, or allergic/irritant reactions.
Lesions inside the mouth, such as canker sores, are ulcers, not blisters, and they are not contagious [NIH].

Types of Lip Blisters

Cold Sores (Fever Blisters)

Cold sores are fluid-filled blisters caused by HSV-1 [CDC]. They often begin with tingling or burning (prodrome) before blisters appear [CDC].
Cold sores commonly form clusters, then crust and heal.
HSV-1 is contagious, including through asymptomatic viral shedding, even when no sore is visible [CDC].

Traumatic Lip Blisters

These occur when the lip is injured by biting, friction from dental appliances, burns, or repetitive irritation.
Trauma-based blisters typically appear as single lesions and are not contagious [Cleveland Clinic].

Allergic or Irritant-Induced Blisters

Allergic contact cheilitis (a T-cell–mediated hypersensitivity reaction) and irritant contact dermatitis (direct skin barrier damage) may develop after exposure to:

• Lip balms, cosmetics, sunscreens
• Toothpaste ingredients or dental materials
• Foods or flavorings
• Metals (nickel-containing items)

These reactions may cause blistering, redness, swelling, peeling, or burning [AAD].

Sun-Induced Blisters

Significant sun exposure can cause blistering of the lips (sunburn blisters), especially in people with fair or sensitive skin.
Sunburn-related blisters are not contagious, but can be painful and slow to heal [Mayo Clinic].

Sun exposure is also a known trigger for cold sore outbreaks in people with HSV-1 [CDC].

Canker Sores (Inside the Mouth)

Canker sores are ulcers, not blisters.
They appear inside the mouth as round white or yellow sores with a red border and are not contagious [NIH].
Included here for differentiation only.

Infection- or STI-Related Lip Lesions

While HSV-1 is the most common viral cause, other infections can produce lip lesions that resemble blisters:

• Impetigo — a bacterial, highly contagious blistering infection often around the mouth [CDC]
• Less commonly, other viral or systemic infections

These require medical evaluation to diagnose accurately.

Causes of Lip Blisters

Common causes include:

• HSV-1 viral infection (cold sores) [CDC]
• Trauma: lip biting, burns, friction, dental appliances [Cleveland Clinic]
• Allergic reactions to cosmetics, balms, foods, or dental materials [AAD]
• Irritant dermatitis from harsh substances (e.g., spicy foods, toothpaste ingredients) [AAD]
• Sunburn, which may blister the lips or trigger cold sores [Mayo Clinic]
• Mucoceles, formed by blockage of minor salivary glands after trauma [Cleveland Clinic]
• Systemic infections or illnesses that cause mouth or lip lesions [CDC]

Accurate identification of the cause guides appropriate treatment.

Symptoms

Symptoms vary by cause but may include:

• A raised, fluid-filled bump on or near the lip
• Tingling or burning before blisters (cold sore prodrome) [CDC]
• Redness, swelling, or tenderness
• Pain during eating, drinking, or speaking
• Clustered blisters (common in cold sores) [CDC]
• Single blisters (common in trauma or irritant injury)
• Crusting or scabbing during healing

Symptoms such as fever, swollen lymph nodes, pus, or spreading redness may indicate infection and require medical care [CDC].

Are Lip Blisters Contagious?

Blisters are contagious only when caused by infectious agents, including:

• HSV-1 cold sores [CDC]
• Impetigo, a bacterial blistering infection [CDC]

Transmission from cold sores can occur through:

• Direct skin or lip contact
• Sharing drinks, straws, utensils, or lip products
• Kissing or oral contact
• Viral shedding when no visible sore is present [CDC]

Blisters from trauma, sunburn, allergies, or irritants are not contagious.

How Long Does a Lip Blister Last?

• Cold sores: generally 7–10 days, sometimes up to 2 weeks [Mayo Clinic]
• Traumatic blisters: usually improve within 3–7 days and heal within 1–2 weeks once irritation stops [Cleveland Clinic]
• Allergic blisters: improve over days to a couple of weeks once the trigger is removed [AAD]
• Sunburn blisters: often heal in about one week, though severe burns may take 1–2 weeks [Mayo Clinic]

Seek medical care if a blister does not improve within 10–14 days.

Diagnosis: How to Tell What Kind of Blister You Have

Key clues:

Location

• Outside lip → trauma, sunburn, cold sore, allergic/irritant reaction
• Inside lip → canker sore, mucocele

Appearance

• Clustered vesicles → cold sores [CDC]
• Single blister → more often trauma or irritation

Symptoms

• Tingling/burning beforehand → suggests HSV-1 [CDC]

A healthcare professional may use:

• Physical exam
• HSV testing (PCR or swab) when viral infection is suspected [CDC]
• Patch or allergy testing for persistent or recurrent reactions [AAD]
• Biopsy for atypical, persistent, or unclear lesions [Cleveland Clinic]

Treatment Options

Home Care

• Do not pop the blister — increases infection risk and slows healing [Cleveland Clinic]
• Apply cool compresses for pain and swelling [Mayo Clinic]
• Avoid spicy, acidic, or abrasive foods
• Keep the area clean; avoid picking or touching

Over-the-Counter Options

• Topical anesthetics (benzocaine, lidocaine) can reduce pain short-term [Cleveland Clinic]
• Barrier ointments (petrolatum) may protect irritated skin and reduce cracking [Mayo Clinic]

Prescription Treatments

• Antiviral medications (acyclovir, valacyclovir, famciclovir) may shorten cold sore duration when started early [CDC]
  These do not eliminate HSV-1 from the body.

Medical Procedures (If Needed)

For recurrent or persistent cyst-like lesions (e.g., mucoceles), treatment options may include:

• Cryotherapy
• Laser therapy
• Surgical excision

These should only be performed by trained medical professionals [Cleveland Clinic].

Prevention

• Use lip sunscreen or protective balms to prevent sun-triggered irritation and burns [Mayo Clinic]
• Avoid known allergens or irritating lip products
• Reduce habits like lip biting
• Practice good hygiene to prevent viral spread
• If cold sores recur due to triggers such as stress or illness, discuss preventive strategies with a clinician [CDC]

When to See a Doctor

Seek medical evaluation if:

• The blister lasts longer than two weeks
• You develop fever with the blister [CDC]
• Redness spreads, pus develops, or infection is suspected
• The blister is extremely painful or interferes with eating
• Blisters recur frequently
• You experience eye irritation during a cold sore outbreak (possible HSV eye involvement) [CDC]