Aciclovir
Is It Right for You?
A complete guide to Aciclovir — what it treats, how it works, dosages, side effects, and when a clinician may prescribe it following an online consultation.
What Is Aciclovir?
Aciclovir (also spelled acyclovir) is an antiviral medication used to treat infections caused by certain strains of the herpes virus family. It is one of the most widely prescribed antivirals in the world and has been in clinical use since the early 1980s, when it represented a landmark advance in the treatment of viral infections.
Unlike antibiotics, which target bacteria, aciclovir works specifically against viruses -- in particular the herpes simplex viruses (HSV-1 and HSV-2), the varicella-zoster virus (VZV) responsible for chickenpox and shingles, and Epstein-Barr virus to a lesser extent. It does not cure herpes infections but significantly reduces the severity, duration, and frequency of outbreaks, and can help prevent transmission.
Aciclovir is available in tablet, capsule, oral suspension, topical cream, and intravenous forms. The oral and topical forms are used for most outpatient conditions. It is suitable for adults and children and is available generically and under the brand name Zovirax.
Oral aciclovir is a prescription-only medication in the UK for most indications. Topical aciclovir cream (for cold sores) is available over the counter. A licensed clinician must assess your symptoms and medical history before oral or high-dose treatment can be prescribed -- which can now be done quickly and conveniently through a telehealth consultation.
What Conditions Is Aciclovir Used For?
Aciclovir is used to treat and manage a range of herpes virus infections:
treatment and suppression of recurrent cold sores caused by herpes simplex virus type 1; oral aciclovir reduces severity and duration when started early.
treatment of primary (first episode) and recurrent genital herpes outbreaks; suppressive therapy to reduce frequency of recurrences and risk of transmission to partners.
treatment in adults, adolescents, immunocompromised patients, and neonates, where the risk of severe disease is higher; most effective when started within 24 hours of rash onset.
treatment to reduce the severity and duration of shingles outbreaks and lower the risk of post-herpetic neuralgia (persistent nerve pain following the rash); most effective when started within 72 hours of rash onset.
Aciclovir treats the symptoms of herpes virus infections and reduces outbreak frequency and severity, but it does not eliminate the virus from the body. The herpes virus remains dormant in nerve tissue and can reactivate at any time. Suppressive therapy can significantly reduce but not completely eliminate the risk of transmission to others.
How Does Aciclovir Work?
Aciclovir is a prodrug -- it is pharmacologically inactive until it is converted into its active form inside virus-infected cells. This selective activation is what makes aciclovir highly effective against herpes viruses while being relatively non-toxic to normal human cells.
The process works in three steps. First, an enzyme called thymidine kinase, produced specifically by herpes viruses, converts aciclovir into aciclovir monophosphate inside infected cells. This first step does not occur efficiently in uninfected human cells -- meaning aciclovir is selectively activated where it is needed most. Second, human cellular enzymes then convert aciclovir monophosphate into the fully active form, aciclovir triphosphate. Third, aciclovir triphosphate acts as a competitive inhibitor and chain terminator of viral DNA polymerase -- the enzyme the virus uses to copy its genetic material. When incorporated into the growing viral DNA strand, aciclovir triphosphate halts replication entirely, preventing the virus from reproducing.
This highly targeted mechanism means aciclovir has an excellent safety profile, as it acts predominantly within virus-infected cells and has minimal impact on normal cellular processes. It is virostatic -- it stops the virus from replicating but relies on the immune system to clear the infection -- rather than directly killing the virus outright.
Dosages & Administration
The correct dose depends on the type and severity of infection, age, weight, and kidney function. Always follow your clinician's instructions. The table below is for general reference only.
Administration Tips
Aciclovir works best when started as early as possible. For cold sores and genital herpes, treatment should ideally begin at the first sign of symptoms -- tingling, itching, or burning -- before blisters appear. For shingles and chickenpox, starting within 72 hours (shingles) or 24 hours (chickenpox) of rash onset gives the best results.
Aciclovir can be taken with or without food. If it causes stomach upset, taking it with a light meal may help.
Space doses evenly throughout the day. For a 5-times-daily regimen, doses should be taken approximately every 4 hours during waking hours (e.g. 6am, 10am, 2pm, 6pm, 10pm), with a longer gap overnight.
Stay well hydrated. Drinking plenty of fluids -- particularly water -- throughout treatment reduces the risk of aciclovir crystallising in the kidneys (crystalluria), especially at higher doses used for shingles and chickenpox.
For topical cream (cold sores): apply to the affected area 5 times daily for 5 days, starting as early as possible in the outbreak. The cream reduces duration but is less effective than oral treatment for frequent or severe outbreaks.
Side Effects of Aciclovir
Oral aciclovir is generally very well tolerated. Serious side effects are uncommon at standard doses but become more relevant at higher doses or in patients with reduced kidney function.
- Nausea or stomach upset
- Vomiting
- Diarrhoea
- Headache
- Dizziness
- Fatigue
- Skin rash or sensitivity to sunlight (photosensitivity)
- Local irritation with topical cream (burning, stinging, itching)
- Neurological effects -- confusion, agitation, tremor, hallucinations, or seizures; more common with intravenous use or in patients with renal impairment receiving high doses
- Renal impairment / acute kidney injury -- caused by crystallisation of aciclovir in renal tubules, particularly at high doses or with inadequate fluid intake; presents as reduced urine output or flank pain
- Severe allergic reaction (anaphylaxis) -- hives, facial or throat swelling, difficulty breathing; rare but requires immediate emergency treatment
- Hepatitis -- elevated liver enzymes and jaundice; rare, more likely with prolonged use or in immunocompromised patients
- Haematological effects -- anaemia, leucopenia, or thrombocytopenia; rare, primarily associated with high-dose or intravenous use
At the higher doses used for shingles (800 mg five times daily) and chickenpox, adequate hydration is essential to prevent aciclovir from crystallising in the kidneys. Patients with pre-existing renal impairment require dose reduction and closer monitoring. Inform your clinician of any kidney conditions before starting treatment.
Drug Interactions
Always disclose all prescription drugs, over-the-counter medicines, vitamins, and supplements to your clinician or pharmacist before starting aciclovir.
Combining aciclovir -- particularly at high doses -- with other medications that affect kidney function increases the risk of renal toxicity. This combination requires careful monitoring of kidney function and adequate hydration.
Cimetidine (an older antacid) reduces renal tubular secretion of aciclovir, raising its plasma levels. Although the interaction is generally modest, it may be clinically relevant in patients with renal impairment.
Aciclovir and mycophenolate compete for renal tubular secretion. Concurrent use can raise plasma levels of both drugs, potentially increasing the risk of side effects from each. Monitoring is advisable in transplant patients.
Important Warnings
At high doses or in patients with impaired kidney function, aciclovir can accumulate to levels that cause serious neurological effects including confusion, hallucinations, agitation, tremor, and seizures. These effects are more common with intravenous use but can occur with high-dose oral therapy. If any neurological symptoms develop during treatment, seek medical attention immediately and do not take further doses until reviewed by a clinician.
Aciclovir can crystallise in the renal tubules, particularly when patients are dehydrated or receiving high doses. This can cause acute kidney injury. Maintaining adequate fluid intake is essential during treatment, especially at doses used for shingles and chickenpox. Patients with pre-existing kidney disease require dose adjustment and are at higher risk.
Aciclovir reduces but does not eliminate the risk of transmitting herpes to sexual partners. Viral shedding can still occur even when no symptoms are present (asymptomatic shedding). The use of condoms and disclosure to partners remains important even during suppressive antiviral therapy.
Patients with HIV, those on immunosuppressive therapy, or those who have received organ or bone marrow transplants are at significantly higher risk of severe herpes virus infections and may require higher doses, longer treatment courses, or intravenous therapy. Aciclovir-resistant strains of herpes virus can also emerge in this population, requiring alternative antiviral treatment.
Speak to a Clinician About Treatment
Getting advice no longer means sitting in a waiting room. Through The GP Service, you can consult with a licensed clinician in minutes — from home, on your lunch break, or wherever works for you. If treatment is clinically appropriate, your clinician can issue a prescription during the consultation. A consultation does not guarantee a prescription.
The treatment you need, when you need it.
Aciclovir FAQs
Yes. A licensed clinician can assess your symptoms via a telehealth consultation and prescribe aciclovir if it is clinically appropriate. For conditions such as genital herpes, shingles, and chickenpox in adults, prescriptions are typically issued the same day. Early treatment is important, so seeking a consultation as soon as symptoms begin gives the best results.
Aciclovir begins working within hours of the first dose. Most patients notice a reduction in pain, itching, and new blister formation within 2--3 days of starting treatment. Existing blisters will still need to heal, but the course of the infection is significantly shortened compared to no treatment.
No. Aciclovir treats the symptoms of herpes outbreaks and can significantly reduce their frequency and severity, but it does not eliminate the herpes virus from the body. The virus remains dormant in nerve tissue and can reactivate. Suppressive therapy (taking aciclovir daily long-term) can reduce outbreaks by up to 70--80% in patients with frequent recurrences.
Suppressive therapy involves taking aciclovir every day -- typically 400 mg twice daily -- to prevent outbreaks rather than treating them when they occur. It is recommended for patients who have 6 or more outbreaks per year, those in relationships with uninfected partners where transmission risk is a concern, or those whose outbreaks have a significant impact on quality of life. Your clinician will discuss whether suppressive therapy is appropriate for you.
Yes. Aciclovir 5% cream for the treatment of cold sores is available over the counter in pharmacies without a prescription. However, oral aciclovir -- which is significantly more effective for recurrent or severe outbreaks -- requires a prescription. If you experience frequent, severe, or genital herpes outbreaks, a consultation with a clinician for oral treatment is strongly recommended.
Yes, but timing is critical. Aciclovir is most effective for shingles when started within 72 hours of the rash appearing. Starting treatment early reduces the severity of the rash, shortens its duration, and most importantly lowers the risk of post-herpetic neuralgia -- the persistent nerve pain that can last for months or years after a shingles outbreak. Do not wait to see if symptoms improve on their own; seek a prescription as soon as possible.
Yes. Aciclovir is used in children for chickenpox, herpes infections, and in neonates with neonatal herpes. Doses are adjusted based on weight and age. Oral aciclovir for chickenpox in otherwise healthy children is not routinely recommended in the UK (as the illness is usually mild), but is indicated in adolescents, adults, immunocompromised children, and those at higher risk of complications.
Aciclovir is not formally licensed for use in pregnancy but has a well-established safety record based on extensive clinical use and registry data. It is commonly used in pregnancy for primary genital herpes, severe varicella, and to suppress herpes near term to prevent neonatal transmission. Always discuss the risks and benefits with your clinician if you are pregnant or planning to become pregnant.