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What Causes Cold Sores and Fever Blisters

Understanding what causes cold sores and fever blisters is the first step towards effective prevention and management. Cold sores are caused by the herpes simplex virus type 1 (HSV-1), which remains dormant in nerve cells and reactivates in response to specific triggers, causing painful blisters around the mouth and lips.

The Herpes Simplex Virus: Primary Cause of Cold Sores

Cold sores and fever blisters are caused by herpes simplex virus type 1 (HSV-1), a highly contagious virus that infects approximately 67% of the global population under age 50 [1]. Once contracted, typically during childhood through close contact with an infected person, the virus travels along nerve pathways and establishes a lifelong latent infection in the trigeminal ganglion.

The initial infection may cause no symptoms, or it can trigger primary herpetic gingivostomatitis — painful mouth ulcers, fever, and swollen lymph nodes [2]. After this first episode, the virus enters a dormant phase within nerve cells, where it remains inactive until specific triggers cause reactivation. This explains why cold sores recur in the same location, as the virus travels back down the same nerve pathway to the skin surface.

HSV-1 transmission occurs through direct contact with infected saliva, skin lesions, or oral secretions. Sharing utensils, lip balm, or kissing someone with an active cold sore significantly increases infection risk. The virus is most contagious when blisters are present, but asymptomatic viral shedding means transmission can occur even without visible symptoms [1].

Common Triggers That Reactivate Cold Sores

While HSV-1 is the underlying cause, various triggers can reactivate the dormant virus and precipitate an outbreak. Stress is one of the most frequently reported triggers, as elevated cortisol levels suppress immune function, allowing the virus to replicate [3]. Both physical stress (illness, surgery, fatigue) and psychological stress (anxiety, emotional distress) can trigger recurrence.

Sunlight exposure, particularly ultraviolet B (UVB) radiation, is a well-documented trigger. UV light can damage skin cells and suppress local immune responses, creating conditions favourable for viral reactivation [3]. Many patients report cold sores developing after holidays in sunny climates or prolonged outdoor exposure without lip protection.

Hormonal fluctuations during menstruation commonly trigger outbreaks in women, whilst fever and upper respiratory infections weaken immune defences, hence the term 'fever blister' [2]. Other recognised triggers include dental procedures, facial trauma, extreme temperatures, and immunosuppressive medications. Identifying your personal triggers allows for targeted prevention strategies.

Individual Trigger Patterns

Trigger patterns vary significantly between individuals. Keeping a cold sore diary can help identify your specific triggers — note when outbreaks occur, what preceded them (stress, illness, sun exposure), and how severe they were. This information enables you to implement preventive measures during high-risk periods, such as applying sunscreen lip balm before sun exposure or starting prophylactic antiviral treatment during times of known stress.

How Cold Sores Develop: The Outbreak Cycle

A cold sore outbreak typically progresses through five distinct stages over 7-10 days. The prodromal stage begins 6-24 hours before visible blisters appear, characterised by tingling, itching, or burning sensations at the site. This is the optimal time to apply antiviral treatment like aciclovir, which can significantly reduce outbreak severity if started early [4].

The blister stage follows, with small fluid-filled vesicles appearing on an erythematous base. These blisters contain millions of viral particles and are highly contagious. Within 48 hours, blisters rupture and weep, forming painful open sores that can merge into larger ulcerated areas. This weeping stage is the most uncomfortable and contagious phase.

The crusting stage begins as the open sores dry and form a yellowish-brown scab. Though less painful, the scab can crack and bleed if the mouth is stretched. Finally, during the healing stage, the scab falls off naturally, revealing pink, regenerated skin underneath. Picking at scabs delays healing and increases scarring risk [2].

When to Start Treatment

Clinical evidence shows that antiviral creams like aciclovir are most effective when applied at the first sign of tingling, ideally within the first 24 hours [4]. At Cured Pharmacy, we stock Numark Cold Sore Cream containing 5% aciclovir from £4.49, which works by blocking viral DNA replication. For frequent or severe outbreaks, oral aciclovir tablets may be prescribed following clinical assessment by our UK prescribers.

Why Some People Get Cold Sores More Frequently

Recurrence frequency varies dramatically between individuals. Approximately 20-40% of HSV-1 infected people experience recurrent cold sores, whilst others never have a second outbreak after the initial infection [1]. Genetic factors influence both susceptibility to infection and recurrence patterns, with certain immune system genes affecting how effectively the body controls latent HSV-1.

Immunocompromised individuals — including those with HIV, organ transplant recipients, or patients on immunosuppressive therapy — experience more frequent and severe outbreaks due to reduced immune surveillance of latent virus [3]. Age also plays a role; recurrence frequency typically decreases with advancing age as the immune system develops more effective control mechanisms.

Chronic stress, poor sleep quality, and nutritional deficiencies (particularly low lysine-to-arginine ratio) have been associated with increased recurrence rates, though evidence remains mixed [2]. Patients experiencing more than six outbreaks annually may benefit from suppressive antiviral therapy, where daily low-dose aciclovir reduces recurrence frequency by up to 70% in clinical trials [4].

Prevention Strategies: Reducing Outbreak Risk

Whilst you cannot eliminate HSV-1 once infected, evidence-based strategies can significantly reduce outbreak frequency. Sun protection is crucial — apply SPF 30+ lip balm before sun exposure, as UV radiation is a potent trigger [3]. During cold weather, protect lips from wind and extreme cold with emollient balms.

Stress management techniques including regular exercise, adequate sleep (7-9 hours nightly), and mindfulness practices help maintain immune function. Some patients find L-lysine supplements (1000mg daily) reduce recurrence frequency, though evidence is inconsistent and NICE does not currently recommend routine supplementation [2].

Avoid sharing personal items like towels, razors, lip balm, or eating utensils. During active outbreaks, refrain from kissing, oral sex, and sharing drinks. Wash hands frequently, especially after touching the affected area. For patients with predictable triggers (such as menstruation or dental work), prophylactic antiviral treatment started 1-2 days before the anticipated trigger can prevent outbreak development [4].

When to Consider Suppressive Therapy

If you experience six or more outbreaks per year, or if outbreaks significantly impact your quality of life, speak with a healthcare professional about suppressive antiviral therapy. This involves taking oral aciclovir daily, which maintains constant antiviral levels in your system and reduces both outbreak frequency and viral shedding. Our UK prescribers at Cured Pharmacy can assess your suitability during a free online consultation.

Treatment Options Available at Cured Pharmacy

Effective cold sore treatment depends on early intervention. Topical aciclovir 5% cream, applied five times daily at the first sign of tingling, reduces healing time by approximately 1-2 days and decreases pain severity [4]. Numark Cold Sore Cream is available from £4.49 and contains the same active ingredient as branded alternatives at a lower cost.

For more severe or frequent outbreaks, oral aciclovir tablets (200mg or 400mg) may be prescribed following clinical assessment. Oral treatment achieves higher systemic antiviral levels than topical creams and is particularly effective for patients with frequent recurrences or immunocompromise. All prescription medications at Cured Pharmacy require completion of a free online consultation with our UK-registered prescribers.

Our superintendent pharmacist, Tarun Kumar (GPhC 2233073), and clinical team ensure all treatments are clinically appropriate and safe for your individual circumstances. We dispense only genuine UK-licensed medicines with transparent upfront pricing and 100% discreet packaging. Treatment is typically dispatched within 24 hours of prescriber approval.

Scientific References

1. James, C., Harfouche, M., Welton, N. J., Turner, K. M., Abu-Raddad, L. J., Gottlieb, S. L., & Looker, K. J. (2020). Herpes simplex virus: global infection prevalence and incidence estimates, 2016. Bulletin of the World Health Organization, 98(5), 315–329. https://doi.org/10.2471/BLT.19.237149
2. Arduino, P. G., & Porter, S. R. (2008). Herpes Simplex Virus Type 1 infection: overview on relevant clinico-pathological features. Journal of Oral Pathology & Medicine, 37(2), 107–121. https://doi.org/10.1111/j.1600-0714.2007.00586.x
3. Whitley, R. J., & Roizman, B. (2001). Herpes simplex virus infections. The Lancet, 357(9267), 1513–1518. https://doi.org/10.1016/S0140-6736(00)04638-9
4. Chi, C. C., Wang, S. H., Delamere, F. M., Wojnarowska, F., Peters, M. C., & Kanjirath, P. P. (2015). Interventions for prevention of herpes simplex labialis (cold sores on the lips). Cochrane Database of Systematic Reviews, 2015(8), CD010095. https://doi.org/10.1002/14651858.CD010095.pub2

Information on this page is for educational purposes only and is not medical advice. All prescription treatments require clinical assessment by a UK-registered prescriber. Always consult a qualified healthcare professional before starting any new medication.