Cold Sores
Symptoms, Causes & Treatment
Understanding cold sores: what causes outbreaks, how to treat them effectively, and how to prevent frequent recurrences with suppressive antiviral therapy.
What Are Cold Sores?
Cold sores (herpes labialis) are small, painful blisters that typically appear on or around the lips, caused by the herpes simplex virus (HSV), most commonly HSV-1. They are extremely common -- an estimated 67% of the global population under 50 carries HSV-1, though many are asymptomatic carriers who never develop visible cold sores.
Once infected, HSV-1 remains dormant in the trigeminal nerve ganglia for life and can reactivate periodically, causing recurrent cold sore outbreaks. The frequency and severity of recurrences vary widely between individuals -- some people have one or two episodes in their lifetime, while others experience frequent outbreaks.
Cold sores are contagious, primarily through direct contact with the blister fluid, but also through asymptomatic viral shedding. The virus can be transmitted to the genitals through oral sex, causing genital herpes (genital HSV-1).
While cold sores are a minor inconvenience for most healthy adults, they can cause serious illness in newborns and immunosuppressed individuals, and can trigger eczema herpeticum in people with active eczema.
What Are the Symptoms of Cold Sores?
Cold sores follow a characteristic progression from tingling prodrome to healed skin over 7–10 days.
Cold sores: tingling, itching, or burning on the lip or around the mouth (prodrome) · Small, fluid-filled blisters that burst and crust over · Painful, red, swollen area on lip · Recurrent episodes triggered by stress, illness, sun, or menstruation · Usually heals within 7–10 days
Cold sore spreading to eye (herpetic keratitis — emergency) · Cold sores in newborns (neonatal herpes) · Eczema herpeticum (cold sore virus spreading in eczema skin) · Cold sores in immunocompromised patients · Sore that does not heal after 3 weeks (exclude lip cancer)
Seek A&E immediately if eye pain or blurred vision develop during a cold sore outbreak (possible HSV keratitis). Cold sores near the eye in anyone with atopic eczema require urgent assessment. Any cold sore in a newborn requires emergency hospital assessment.
What Causes Cold Sores?
Cold sores are caused by herpes simplex virus type 1 (HSV-1), which establishes lifelong latency in the trigeminal ganglion after primary infection.
HSV-1 causes most cold sores via primary infection in childhood or adolescence. After primary infection, HSV-1 establishes latency in the trigeminal ganglion and reactivates periodically to cause labial herpes.
Melanin is produced by melanocytes in response to UV radiation and inflammation. Excess melanin deposition causes post-inflammatory hyperpigmentation after acne, eczema, or injury, particularly in darker skin tones.
Contact dermatitis occurs when skin comes into contact with an irritant (irritant contact dermatitis) or a specific allergen (allergic contact dermatitis via type IV delayed hypersensitivity). Common allergens: nickel, latex, fragrances, preservatives.
Common eczema triggers include wool, synthetic fabrics, dust mites, pet dander, pollen, mould, soaps, detergents, food allergens (in children), and emotional stress. Identifying and avoiding personal triggers reduces flare frequency.
Psoriasis is an immune-mediated chronic inflammatory skin disease. T-cell activation drives rapid keratinocyte proliferation (3–5 days instead of 28–30), producing the characteristic thickened plaques.
Multiple psoriasis triggers are recognised: streptococcal throat infection (especially guttate psoriasis), medications (lithium, beta-blockers, antimalarials, NSAIDs), stress, trauma (Koebner phenomenon), alcohol, and smoking.
Key Risk Factors
How Are Cold Sores Diagnosed?
Cold sores are diagnosed clinically on characteristic appearance and location. VZV PCR swab confirms HSV if diagnosis is uncertain. Blood HSV serology confirms past infection and can identify primary vs recurrent episodes.
How Are Cold Sores Treated?
Topical antivirals reduce duration modestly. Oral antivirals are significantly more effective and should be prescribed for frequent outbreaks.
Supportive Measures
Apply topical aciclovir or penciclovir cream every 2 hours while awake at the first sign of tingling. Avoid kissing and oral-genital contact during outbreaks. Do not share lip products, cups, or cutlery. Keep the sore moist. Take paracetamol for pain.
Reducing Frequent Cold Sore Outbreaks
If cold sores occur 6 or more times per year, daily suppressive therapy with aciclovir 400mg twice daily or valaciclovir 500mg daily significantly reduces frequency. It is safe for long-term use. Trigger management is equally important.
When Should You Seek Medical Advice for Cold Sores?
Cold sore with eye pain, redness, or blurred vision (HSV keratitis) — attend A&E immediately · Cold sore with confusion, stiff neck, or photophobia (possible HSV encephalitis) — call 999.
You have 6 or more cold sore outbreaks per year · You have eczema and develop a cold sore (eczema herpeticum risk) · Cold sore is near or affecting the eye · You have a cold sore and are around a newborn baby.
How Can You Reduce Cold Sore Outbreaks?
Cold sores cannot be prevented entirely, but their frequency, severity, and transmission risk can be significantly reduced.
Keep feet clean and dry — especially between the toes. Wear flip-flops in communal showers. Change socks daily. Use antifungal powder in sports shoes. Treat athlete's foot promptly to prevent nail spread.
Apply SPF 30+ sunscreen daily to prevent UV triggering cold sore reactivation. Cold sores activated by UV are particularly common in skiers, outdoor workers, and sun-holiday travellers.
Frequent, effective milk removal is the most important mastitis prevention. Feed or express every 2–3 hours; never skip feeds. Get breastfeeding support from a midwife, health visitor, or lactation consultant early — before problems develop.
Ensure your baby latches on deeply and correctly. A poor latch causes nipple trauma (cracks), which are an entry point for bacteria. Ask for a latch assessment if breastfeeding is painful.
Seek breastfeeding support immediately if you notice breast hardness, redness, or pain. Early identification and treatment of blocked ducts prevents progression to mastitis and abscess.
SPF 30+ sunscreen daily is the single most effective measure against all types of hyperpigmentation, including melasma, PIH, and solar lentigines. UV exposure stimulates melanin even on cloudy days. Apply year-round to face and neck.
Speak to a Clinician About Cold Sores
Through The GP Service, a licensed clinician can prescribe oral antivirals for frequent cold sores and arrange daily suppressive therapy for those with recurrent episodes.
Expert clinical advice, when you need it.
Cold Sores FAQs
Cold sores are caused by herpes simplex virus type 1 (HSV-1). The virus is acquired through close contact (kissing, sharing utensils) and remains latent in nerve tissue for life. Triggers such as illness, sun exposure, stress, or hormonal changes can reactivate it.
Cold sores are most contagious during the tingling and blistering stages. Avoid kissing, sharing lip products, cups, or cutlery during an outbreak. The virus can be transmitted even without a visible sore, but risk is highest when a sore is present.
Topical aciclovir or penciclovir cream started at the first sign of tingling can reduce the duration and severity of a cold sore. Oral antivirals (aciclovir, valaciclovir) are more effective and recommended for frequent or severe cold sores.
Most cold sores heal in 7–10 days without treatment. With antiviral treatment started early, duration may be shortened by 1–2 days. The tingling prodrome typically lasts 1–2 days before the sore appears.
If you have 6 or more cold sore episodes per year, daily suppressive antiviral therapy (aciclovir or valaciclovir) can significantly reduce frequency. This is available on prescription and is safe for long-term use.
Oral HSV-1 can be transmitted to a partner’s genitals through oral sex. This causes genital herpes with the same symptoms but caused by HSV-1. Using barrier protection during oral sex reduces transmission risk.
Cold sores in or near the eye (ocular herpes) require urgent assessment. HSV keratitis can cause corneal scarring and permanent vision loss. If you develop eye pain, redness, or blurred vision during a cold sore outbreak, attend A&E immediately.
Avoid exposing active cold sores to newborns — neonatal herpes can be life-threatening. If you have a cold sore and are around newborns, wash hands thoroughly, avoid kissing the baby, and do not share items that touch your mouth.