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Aciclovir Tablets Side Effects: Complete Guide

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Aciclovir Tablets Side Effects: Complete Guide

Published on: June 03, 2026

Understanding aciclovir tablets side effects UK is essential for safe and effective cold sore treatment. Aciclovir is a well-established antiviral medication licensed by the MHRA for treating herpes simplex infections, and whilst most patients tolerate it well, knowing what to expect helps you use it confidently and recognise when to seek medical advice.

How Aciclovir Tablets Work and Why Side Effects Occur

Aciclovir is a nucleoside analogue antiviral that selectively targets cells infected with herpes simplex virus (HSV). Once inside infected cells, it's converted into its active form by viral enzymes, then inhibits viral DNA polymerase to stop the virus replicating [1]. This targeted mechanism means aciclovir has minimal effect on healthy human cells, which is why serious side effects are uncommon.

Most side effects occur because aciclovir is processed through the kidneys and can occasionally affect gastrointestinal function. The 400mg and 800mg tablet strengths commonly prescribed for cold sores contain sufficient active ingredient to suppress viral replication, but this systemic exposure means some patients experience mild reactions [2]. Understanding this helps distinguish between expected minor effects and symptoms requiring medical attention.

Common Aciclovir Side Effects: What to Expect

Clinical trials show that most aciclovir side effects are mild and resolve without intervention. The most frequently reported reactions affect fewer than 1 in 10 patients and include headache, nausea, dizziness, and mild gastrointestinal upset such as diarrhoea or stomach discomfort [1][3]. These symptoms typically appear within the first few days of treatment and diminish as your body adjusts to the medication.

Skin reactions are also relatively common, with some patients reporting mild rash or itching. Fatigue and general malaise affect a small proportion of users, particularly during the first treatment course. It's worth noting that many cold sore sufferers already feel unwell due to the viral infection itself, so distinguishing between viral symptoms and medication side effects can be challenging initially.

Managing Common Side Effects at Home

If you experience mild nausea, taking aciclovir tablets with food often helps, though the medication can be taken with or without meals. Staying well-hydrated is particularly important as it supports kidney function and may reduce headache intensity. For mild headaches, standard over-the-counter paracetamol or ibuprofen is generally safe to use alongside aciclovir, but always check with your pharmacist if you're taking other medications [2].

Uncommon and Rare Aciclovir Side Effects

Uncommon side effects, affecting between 1 in 100 and 1 in 1,000 patients, include more pronounced gastrointestinal symptoms such as vomiting, increased sensitivity to sunlight (photosensitivity), and urticaria (hives). Some patients report hair loss during prolonged treatment courses, though this typically reverses after stopping the medication [3].

Rare but serious side effects require immediate medical attention. These include signs of kidney problems such as reduced urine output, blood in urine, or lower back pain; neurological symptoms like confusion, hallucinations, tremor, or seizures; and severe allergic reactions characterised by facial swelling, difficulty breathing, or widespread rash [1][4]. Whilst these reactions are uncommon, awareness ensures prompt recognition and appropriate medical response.

Risk Factors for Serious Side Effects

Patients with pre-existing kidney disease, elderly individuals, and those taking high doses face elevated risk of neurological side effects. Dehydration significantly increases the likelihood of kidney-related complications, which is why maintaining adequate fluid intake throughout treatment is emphasised by UK prescribers. Your clinical assessment before receiving aciclovir helps identify these risk factors and ensures appropriate monitoring.

Frequency Side Effects Action Required
Common (up to 1 in 10) Headache, nausea, dizziness, diarrhoea, mild rash Monitor; usually resolve without intervention
Uncommon (up to 1 in 100) Vomiting, photosensitivity, urticaria, hair loss Contact pharmacist for advice
Rare (up to 1 in 1,000) Kidney problems, confusion, tremor, severe allergic reaction Seek urgent medical attention
Very rare (less than 1 in 10,000) Seizures, hepatitis, blood disorders Emergency medical assessment required

Aciclovir 400mg Side Effects vs Other Strengths

The 400mg strength commonly prescribed for cold sore treatment typically produces fewer side effects than the 800mg dose used for shingles or genital herpes. Lower doses mean reduced systemic exposure whilst still providing effective viral suppression for uncomplicated cold sores [2]. Standard cold sore treatment involves 400mg taken five times daily for five days, which balances efficacy against tolerability.

Patients prescribed higher doses or longer treatment courses should be particularly vigilant about hydration and monitoring for side effects. The 200mg strength is sometimes used for suppressive therapy in patients with frequent recurrences, and this lower dose is associated with even fewer adverse reactions. Your UK prescriber will select the appropriate strength based on your infection severity, medical history, and previous treatment response.

When to Seek Medical Advice About Side Effects

Contact your GP or pharmacist if you experience persistent vomiting that prevents you from keeping tablets down, severe or worsening headache, unusual bruising or bleeding, or yellowing of skin or eyes. These symptoms may indicate complications requiring dose adjustment or alternative treatment [3][4].

Seek emergency medical attention immediately if you develop signs of severe allergic reaction (anaphylaxis) including difficulty breathing, rapid swelling of face or throat, or widespread skin blistering. Similarly, neurological symptoms such as confusion, seizures, loss of consciousness, or severe tremor warrant urgent assessment. At Cured Pharmacy, our clinical team is available on (+44) 116 4646009 during business hours for advice about ongoing treatment, though emergency symptoms always require NHS urgent care or A&E attendance.

Reporting Side Effects to the MHRA

The UK's Yellow Card scheme allows patients and healthcare professionals to report suspected side effects directly to the Medicines and Healthcare products Regulatory Agency (MHRA). Reporting helps identify previously unknown reactions and contributes to ongoing medication safety monitoring. You can submit reports online at yellowcard.mhra.gov.uk or via the Yellow Card app, and your report remains confidential whilst contributing valuable safety data.

Minimising Aciclovir Side Effects: Practical Guidance

Starting treatment at the first sign of cold sore tingling (prodrome) not only improves antiviral efficacy but may reduce side effect risk by limiting the treatment duration needed [2]. Taking tablets at evenly spaced intervals maintains consistent blood levels and reduces peaks that might trigger nausea or dizziness. Setting phone reminders helps maintain the five-times-daily schedule without disrupting sleep.

Drinking at least two litres of water daily supports kidney function and reduces the risk of crystalluria (crystal formation in urine), a rare complication associated with high-dose aciclovir. Avoiding alcohol during treatment may reduce nausea and dizziness, though moderate consumption isn't strictly contraindicated. If you're taking other medications, always inform your prescriber as certain drug interactions can increase side effect risk.

Aciclovir Cream vs Tablets: Side Effect Comparison

Topical aciclovir cream, such as Numark Cold Sore Cream available from £4.49 at Cured Pharmacy, produces minimal systemic side effects because very little medication enters the bloodstream. Local application site reactions like mild stinging or dryness are the main concerns with cream formulations. For patients who experience troublesome side effects from oral tablets, topical treatment offers an alternative for mild cold sore outbreaks, though tablets remain more effective for severe or frequent infections [1].

Scientific References

  1. 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]
  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), 1500–1507. https://doi.org/10.1086/374877 [accessed 13 August 2026]
  3. Drugs and Lactation Database (LactMed). (2006). Acyclovir. National Library of Medicine (US). Bethesda (MD): National Library of Medicine. https://www.ncbi.nlm.nih.gov/books/NBK501090/ [accessed 13 August 2026]
  4. Brigden, D., & Whiteman, P. (1983). The mechanism of action, pharmacokinetics and toxicity of acyclovir—a review. Journal of Infection, 6(1), 3–9. https://doi.org/10.1016/S0163-4453(83)90866-3 [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. If you experience severe side effects, seek immediate medical attention or contact NHS 111.

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Faq

What are the most common aciclovir tablets side effects?
The most common side effects include headache, nausea, dizziness, and mild gastrointestinal upset, affecting fewer than 1 in 10 patients. These typically resolve within a few days and rarely require stopping treatment.
Can aciclovir tablets cause kidney problems?
Kidney problems are rare but can occur, particularly in patients with pre-existing kidney disease, elderly individuals, or those who become dehydrated. Maintaining good hydration throughout treatment significantly reduces this risk.
Should I take aciclovir tablets with food?
Aciclovir can be taken with or without food, but taking tablets with meals may reduce nausea in sensitive individuals. The medication's effectiveness is not affected by food intake.
How long do aciclovir side effects last?
Most common side effects resolve within 2-3 days as your body adjusts to the medication. Side effects typically disappear completely within 24-48 hours of finishing the treatment course.
Are aciclovir 400mg side effects different from 800mg?
Lower doses like 400mg typically produce fewer and milder side effects compared to 800mg strengths. The 400mg dose commonly used for cold sores balances effectiveness with tolerability.
Can I drink alcohol while taking aciclovir tablets?
Alcohol is not strictly contraindicated with aciclovir, but it may worsen side effects like nausea and dizziness. Limiting alcohol intake during treatment is generally advisable.
What should I do if I experience severe aciclovir side effects?
Stop taking the medication and seek immediate medical attention if you develop difficulty breathing, facial swelling, severe rash, confusion, seizures, or signs of kidney problems such as reduced urination or blood in urine.
Is aciclovir cream safer than tablets for avoiding side effects?
Topical aciclovir cream produces minimal systemic side effects because very little enters the bloodstream, causing mainly local reactions like mild stinging. However, tablets are more effective for severe or recurrent cold sores.
How much does Aciclovir cost at Cured Pharmacy?
Aciclovir tablets at Cured Pharmacy are competitively priced, with costs varying depending on dosage strength and pack size. Our online consultation service is completely free, and we offer transparent pricing with no hidden fees for UK customers.
Can I get Aciclovir tablets without seeing a doctor in person?
Yes, Cured Pharmacy offers a convenient online consultation service where our UK-registered prescribers can assess your suitability for Aciclovir tablets. If approved, your prescription is issued digitally and your medication dispatched discreetly to your address.
How quickly can Aciclovir be delivered in the UK?
Cured Pharmacy offers next-day delivery across the UK when you order before our cut-off time. All Aciclovir tablets are dispatched in discreet, unmarked packaging from our registered UK pharmacy to ensure your privacy.
Are Aciclovir tablets safe for long-term use?
Aciclovir is generally considered safe for long-term suppressive therapy in patients with recurrent herpes infections when prescribed by a healthcare professional. Our pharmacists at Cured Pharmacy can provide guidance on safe usage and monitor your treatment through regular consultations.
What should I do if I experience side effects from Aciclovir tablets?
If you experience mild side effects like headache or nausea, these often resolve on their own. However, if you develop serious side effects such as unusual bruising, confusion, or allergic reactions, stop taking Aciclovir immediately and contact Cured Pharmacy or seek urgent medical attention.
Tarun Kumar, Prescribing Pharmacist

Medically reviewed by

Tarun Kumar, Prescribing Pharmacist (GPhC 2233073)

Reviewed on: June 03, 2026

Last reviewed: 13 August 2026

Last updated on 13 August 2026.

Review History

Our experts continually monitor new findings in health and medicine, and we update our articles when new information becomes available.

Why this page was updated on 13 August 2026

Content checked and updated as part of our periodic review, to ensure accuracy and currentness.

Current version (13 August 2026)

Edited by: The Editorial Team

Medically reviewed by:

Tarun Kumar, GPhC No. 2233073, Prescribing Pharmacist


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