Our Pricing

Cold Sore Tablets Dosage UK: Complete Prescribing Guide

Published on: June 03, 2026

Understanding cold sore tablets dosage UK guidelines is essential for effective treatment of herpes simplex virus outbreaks. At Cured Pharmacy, our UK-registered clinical team provides evidence-based guidance on prescription aciclovir tablets, helping you access the right dosage through a free online consultation with transparent upfront pricing from £4.49.

Standard Aciclovir Tablet Dosage for Cold Sores

The standard aciclovir tablets dosage for cold sores in the UK is 200mg taken five times daily for five days, as recommended by NICE guidelines [1]. This dosing schedule maintains therapeutic blood levels throughout the day, suppressing viral replication most effectively when treatment begins at the first sign of tingling or prodromal symptoms.

Clinical trials demonstrate that early initiation of aciclovir 200mg five times daily reduces healing time by an average of 1.5 days compared to placebo, with the greatest benefit observed when treatment starts within 48 hours of symptom onset [2]. Some UK prescribers may recommend an alternative regimen of 400mg three times daily, which offers similar efficacy with improved convenience for patients who struggle with the five-times-daily schedule.

Treatment duration is typically five days for most patients, though your UK prescriber may extend this to ten days if lesions have not fully healed or if you experience severe recurrent outbreaks [1]. All prescription cold sore tablets require clinical assessment by a UK-registered prescriber to determine the appropriate dosage based on your medical history, kidney function, and frequency of outbreaks.

How Aciclovir Tablets Work Against Herpes Simplex Virus

Aciclovir is a nucleoside analogue antiviral that selectively targets herpes simplex virus type 1 (HSV-1), the primary cause of cold sores [3]. Once absorbed, aciclovir is converted by viral thymidine kinase into its active triphosphate form, which inhibits viral DNA polymerase and terminates viral DNA chain elongation, effectively stopping viral replication.

This selective mechanism means aciclovir concentrates in virus-infected cells at levels 40 to 100 times higher than in uninfected cells, minimising systemic side effects whilst maximising antiviral activity [3]. The drug's half-life of approximately 2.5 hours necessitates multiple daily doses to maintain therapeutic concentrations, explaining why the standard UK dosing schedule requires five administrations throughout waking hours.

Oral aciclovir tablets achieve systemic distribution, making them more effective than topical creams for moderate to severe outbreaks or for patients experiencing frequent recurrences. Bioavailability of oral aciclovir ranges from 15% to 30%, with peak plasma concentrations reached within 1.5 to 2 hours after administration [2].

Tablets vs Topical Creams: Clinical Effectiveness

Whilst topical aciclovir cream like Numark Cold Sore Cream (available from £4.49) may be suitable for very mild or infrequent cold sores, oral aciclovir tablets demonstrate superior clinical outcomes for most patients. Meta-analyses show oral antiviral therapy reduces healing time by approximately 0.5 to 1.5 days more than topical treatments, with greater reductions in pain duration and viral shedding [4].

Your UK prescriber will recommend tablets over creams if you experience cold sores more than twice yearly, have immunosuppression, or develop lesions that are painful, widespread, or affect eating and drinking. Prescription tablets are also preferred for suppressive therapy in patients with very frequent outbreaks (six or more per year).

Dosage Adjustments for Specific Patient Groups

Patients with impaired kidney function require dosage modifications because aciclovir is primarily eliminated renally [1]. For those with creatinine clearance between 10-25 mL/min, the standard 200mg dose is typically reduced to 200mg twice daily, whilst patients with severe renal impairment (creatinine clearance below 10 mL/min) may receive 200mg once daily. Your UK prescriber will assess your kidney function during the online consultation to determine if dose adjustment is necessary.

Elderly patients may also require modified dosing due to age-related decline in renal function, even when routine blood tests appear normal. MHRA guidance recommends ensuring adequate hydration in older adults taking aciclovir to reduce the risk of crystalluria, though this is rare at standard cold sore treatment doses [5].

Pregnant and breastfeeding women can generally use aciclovir tablets when clinically necessary, as the drug has been used extensively during pregnancy with no evidence of increased birth defects [6]. However, your UK prescriber will carefully weigh the benefits against potential risks and may recommend the lowest effective dose for the shortest duration necessary to control the outbreak.

Paediatric Dosing Considerations

Children aged over two years with cold sores may be prescribed aciclovir tablets at doses calculated based on body weight, typically 20mg/kg four times daily, up to a maximum of 200mg per dose [1]. Paediatric prescribing requires careful assessment by a UK prescriber, as cold sores in young children may occasionally indicate more serious herpes simplex infections requiring different management.

Suppressive Therapy Dosing for Frequent Cold Sores

Patients experiencing six or more cold sore outbreaks annually may benefit from suppressive aciclovir therapy, which involves taking a lower daily dose continuously to prevent recurrences [2]. The standard suppressive dosage in the UK is 400mg twice daily, taken every day for a period determined by your prescriber, typically six to twelve months initially.

Clinical studies show suppressive aciclovir therapy reduces outbreak frequency by approximately 75% in patients with frequent recurrent herpes simplex labialis [4]. After six to twelve months of suppressive treatment, your UK prescriber will typically recommend a treatment break to reassess your natural outbreak frequency, as some patients experience a sustained reduction in recurrences even after stopping prophylactic therapy.

Suppressive therapy requires ongoing monitoring by a UK healthcare professional and is only prescribed after careful assessment of outbreak frequency, impact on quality of life, and absence of contraindications. This approach is particularly valuable for patients whose cold sores significantly affect their professional or social activities, or those who develop severe or prolonged lesions with each outbreak.

Managing Missed Doses and Treatment Timing

If you miss a dose of aciclovir tablets, take it as soon as you remember, then continue with your regular schedule [5]. However, if it's nearly time for your next dose, skip the missed dose entirely and resume your normal dosing interval—never double up to compensate for a forgotten dose, as this may increase the risk of side effects without improving efficacy.

The effectiveness of cold sore tablets depends critically on treatment timing. Starting aciclovir at the prodromal stage (when you first feel tingling, itching, or burning) before visible lesions appear produces the best clinical outcomes [2]. Patients who begin treatment within the first 24 hours of symptom onset experience significantly shorter healing times compared to those who delay treatment until blisters have fully formed.

For maximum convenience with the five-times-daily regimen, many patients find it helpful to take doses at approximately four-hour intervals during waking hours—for example, at 7am, 11am, 3pm, 7pm, and 11pm. Setting phone reminders can help maintain adherence, which is essential for achieving optimal antiviral suppression throughout the five-day treatment course.

What to Do If Symptoms Persist

If your cold sore has not significantly improved after five days of treatment, or if lesions worsen despite taking aciclovir tablets as prescribed, contact your UK prescriber for reassessment. Persistent or severe symptoms may indicate antiviral resistance (rare but possible), secondary bacterial infection, or an alternative diagnosis requiring different management. Never extend your course of aciclovir beyond the prescribed duration without consulting a healthcare professional.

Side Effects and Safety Considerations

Aciclovir tablets are generally well-tolerated, with most patients experiencing no side effects at standard cold sore treatment doses [3]. The most commonly reported adverse effects include headache (affecting approximately 13% of patients), nausea (occurring in about 5% of users), and mild gastrointestinal upset such as diarrhoea or abdominal discomfort [5].

Serious side effects are rare but require immediate medical attention. These include signs of allergic reaction (rash, difficulty breathing, swelling of face or throat), unusual bruising or bleeding, confusion, hallucinations, or seizures [5]. Patients with reduced kidney function are at slightly higher risk of neurological side effects, which is why dose adjustment is essential in this population.

Drug interactions with aciclovir are uncommon, but you should inform your UK prescriber during your online consultation about all medications you're taking, including over-the-counter medicines and supplements. Particular caution is needed with drugs that affect kidney function, such as certain diuretics or NSAIDs, as these may increase aciclovir blood levels [1]. Adequate hydration whilst taking aciclovir tablets helps support kidney function and reduces the already-low risk of crystalluria.

Scientific References

1. National Institute for Health and Care Excellence. (2023). Herpes simplex – oral. NICE Clinical Knowledge Summary. https://cks.nice.org.uk/topics/herpes-simplex-oral/ [accessed 13 August 2026]
2. Spruance, S. L., et al. (2003). Peroral famciclovir in the treatment of experimental ultraviolet radiation-induced herpes simplex labialis: a double-blind, dose-ranging, placebo-controlled, multicenter trial. Journal of Infectious Diseases, 187(10), 1608-1617. https://doi.org/10.1086/374800 [accessed 13 August 2026]
3. De Clercq, E., & Field, H. J. (2006). Antiviral prodrugs – the development of successful prodrug strategies for antiviral chemotherapy. British Journal of Pharmacology, 147(1), 1-11. https://doi.org/10.1038/sj.bjp.0706446 [accessed 13 August 2026]
4. Chi, C. C., et al. (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 [accessed 13 August 2026]
5. Medicines and Healthcare products Regulatory Agency. (2024). Aciclovir 200mg and 400mg tablets: Summary of Product Characteristics. Electronic Medicines Compendium. https://www.medicines.org.uk/emc/ [accessed 13 August 2026]
6. Pasternak, B., & Hviid, A. (2010). Use of acyclovir, valacyclovir, and famciclovir in the first trimester of pregnancy and the risk of birth defects. JAMA, 304(8), 859-866. https://doi.org/10.1001/jama.2010.1206 [accessed 13 August 2026]

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.