Zovirax (Aciclovir)

What is Zovirax?

Zovirax (aciclovir) is the original antiviral medicine for herpes simplex and herpes zoster infections. Developed in the 1970s, aciclovir was a landmark in antiviral pharmacology and established the template for the class of nucleoside analogue antivirals. It is authorised under product- and country-specific conditions, listed on the WHO Model List of Essential Medicines, and remains a cornerstone treatment for herpes virus infections.

In sexual health, oral aciclovir tablets are used to treat genital herpes (caused by herpes simplex virus types 1 and 2) — both for managing active outbreaks and as daily suppressive therapy to reduce the frequency of recurrences. A transmission benefit established for another antiviral must not be assumed.

Zovirax is available in oral tablet (200 mg, 400 mg, 800 mg), topical cream (5%), eye ointment, and intravenous formulations. For genital herpes, oral tablets are the primary therapeutic form.

How does Zovirax work?

Aciclovir is a synthetic nucleoside analogue (structurally similar to the natural nucleoside guanosine) that inhibits herpes virus DNA replication through a highly selective mechanism:

1. Selective activation by viral thymidine kinase: Aciclovir enters cells. In cells infected with herpes simplex virus (HSV) or varicella-zoster virus (VZV), viral thymidine kinase (TK) — present only in infected cells — phosphorylates aciclovir to aciclovir monophosphate. Uninfected human cells lack the viral TK and convert aciclovir very inefficiently.
2. Further phosphorylation: Cellular kinases convert aciclovir monophosphate to aciclovir triphosphate, the active antiviral form.
3. Viral DNA polymerase inhibition: Aciclovir triphosphate competes with the natural substrate (deoxyguanosine triphosphate) for incorporation into the viral DNA chain by the viral DNA polymerase. Once incorporated, aciclovir causes obligatory chain termination — the DNA strand cannot extend further.
4. Viral replication halted: With DNA synthesis blocked, herpes virus cannot replicate within infected cells.

This mechanism is highly selective because the activating enzyme (viral TK) is present only in virus-infected cells. Uninfected human cells are largely unaffected, which accounts for aciclovir’s favourable safety profile.

Why oral bioavailability is limited: Aciclovir is incompletely absorbed from the gut (bioavailability approximately 15–30%), which is why valacyclovir (Valtrex), the prodrug of aciclovir, achieves substantially higher blood levels and requires less frequent dosing. Despite lower bioavailability, oral aciclovir achieves therapeutic tissue concentrations sufficient for treating herpes infections.

Who is Zovirax for?

Sexual health indications:

• First episode genital herpes: Reduces severity and duration of the primary outbreak
• Recurrent genital herpes (episodic): Started at the earliest sign of recurrence to shorten outbreak
• Suppressive therapy: Daily treatment to reduce frequency of outbreaks; transmission can still occur

Other indications:

• Orolabial herpes (cold sores): Oral tablets or topical cream (topical cream is most widely used for cold sores)
• Herpes zoster (shingles): Reduces duration and severity if started within 72 hours of rash onset
• Varicella (chickenpox) in at-risk adults: Under medical guidance
• Herpes encephalitis and neonatal herpes: Intravenous aciclovir under specialist management

Zovirax is not effective against:

• HIV or other retroviruses
• HPV (genital warts)
• Bacterial or fungal infections
• Cytomegalovirus (CMV) — aciclovir has limited activity; specialist antivirals are used

Dosage

Oral Zovirax Tablets

Aciclovir tablets can be taken with or without food. Adequate hydration is important during treatment to avoid aciclovir crystallisation in the kidney tubules (rare but possible at high doses).

Note on dosing frequency: The five-times-daily dosing schedule for episodic treatment is the main practical disadvantage of aciclovir compared to valacyclovir (twice daily) or famciclovir (twice daily). For suppressive therapy, the twice-daily 400 mg regimen is more manageable.

Topical Zovirax Cream 5%

For cold sores: apply 5 times daily at approximately 4-hour intervals for 5 days. Begin treatment as early as possible (at the tingling/prodrome stage). Not recommended as primary treatment for genital herpes.

Side effects

Oral aciclovir at standard doses is very well tolerated.

Common side effects (at standard doses)

• Nausea
• Headache
• Dizziness
• Fatigue
• Diarrhoea

Less common side effects

• Vomiting
• Abdominal pain
• Skin rash or photosensitivity
• Elevated liver enzymes (usually mild and reversible)

Serious side effects (more common at high doses or with renal impairment)

• Acute kidney injury: Aciclovir can crystallise in renal tubules at high doses or when inadequately hydrated. Ensure good fluid intake throughout treatment.
• Neurological effects: Confusion, agitation, tremors, hallucinations, somnolence — reported particularly in elderly patients or those with renal impairment receiving high doses. Dose reduction reduces risk.
• Thrombotic thrombocytopenic purpura / haemolytic uraemic syndrome (TTP/HUS): Very rare, reported in severely immunocompromised patients at high doses.
• Serious allergic reactions: Including anaphylaxis and angioedema (very rare)

Topical cream

Local skin reactions at the application site (mild stinging, burning) are uncommon.

Zovirax vs alternative herpes antivirals

Zovirax vs Valtrex: Both are clinically effective for genital herpes. Valtrex’s higher bioavailability allows far more convenient dosing (twice daily vs five times daily for episodic treatment, once daily vs twice daily for suppression). Aciclovir (Zovirax) is often less expensive and may be preferred in price-sensitive settings or where once-daily suppressive therapy is not required.

Zovirax in pregnancy: Aciclovir has the largest safety dataset of any herpes antiviral in pregnancy. Registry data on aciclovir exposure in pregnancy has not shown increased risk of adverse fetal outcomes, making oral aciclovir the antiviral of choice when treating herpes in pregnancy (under medical supervision).

Online assessment for Zovirax (Aciclovir)

Oral Zovirax (aciclovir tablets) is a prescription-only medicine (POM) in most EU member states. It cannot legally be dispensed without a valid prescription from a licensed medical professional.

Prescrivia operates as a technology intermediary: we do not prescribe medicines, employ doctors, or sell medicines. Our platform connects patients with independent doctors who can conduct confidential online health assessments.

Availability of an online assessment, prescribing, dispensing and delivery depends on the medicine, country and applicable law. Spain prohibits distance sale of prescription medicines; a catalogue entry or cross-border prescription does not establish a right to delivery. An in-person examination or testing may be required.

The process, where legally and clinically available:

1. Complete a health assessment: Describe your symptoms, history of herpes episodes, relevant medical history (especially kidney function), and any current medications.
2. Doctor review: An independent doctor reviews your assessment. If aciclovir is clinically appropriate, they may issue a prescription.
3. Pharmacy fulfilment: The prescription is sent to a pharmacy partner to assess dispensing and any supply under the applicable country rules.

Important: Prescrivia does not guarantee that a prescription will be issued. All prescribing decisions are made independently by qualified medical professionals.

Important safety information

Tell your doctor before taking Zovirax if you:

• Have kidney disease (dose reduction is required — aciclovir is renally excreted)
• Are elderly (higher risk of neurological side effects; ensure adequate hydration)
• Are pregnant or breastfeeding (aciclovir is generally considered the preferred antiviral in pregnancy; discuss with your doctor)
• Are taking nephrotoxic medicines such as ciclosporin, or medicines such as probenecid that reduce aciclovir clearance
• Are immunocompromised (HIV infection, transplant medication, cancer treatment) — higher doses may be needed; specialist management is recommended

Maintain good hydration: Drink adequate water throughout treatment to prevent aciclovir crystallisation in the kidneys, particularly at higher doses.

Herpes transmission: Transmission can occur without visible lesions. Condoms reduce but do not eliminate risk; the transmission reduction established for valacyclovir in a specific population must not be assumed for aciclovir. Disclosure to sexual partners is important to support informed decision-making about sexual health.

No cure: Aciclovir and other herpes antivirals control viral replication and alleviate symptoms but do not eliminate the herpes virus. The virus remains latent in sensory nerve ganglia and may reactivate throughout a person’s lifetime.

Sources

• Product information / clinical reference 1
• Product information / clinical reference 2
• Product information / clinical reference 3

Medical information on this page is based on the following sources:

• World Health Organization (WHO). Herpes simplex virus — Fact sheet. who.int
• World Health Organization (WHO). WHO Model List of Essential Medicines. 22nd edition, 2021.
• Elion GB. The purine path to chemotherapy (Nobel Lecture). Angewandte Chemie (International Ed.). 1989.
• ACOG (American College of Obstetricians and Gynecologists). Management of Genital Herpes in Pregnancy. Practice Bulletin No. 220. 2020.

The content-edit date does not certify a clinical review. It is provided for informational purposes only and does not constitute medical advice. Always consult a qualified healthcare professional for advice relevant to your individual health circumstances.