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 approved by the European Medicines Agency (EMA), 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 and risk of transmission.
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:
- 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.
- Further phosphorylation: Cellular kinases convert aciclovir monophosphate to aciclovir triphosphate, the active antiviral form.
- 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.
- 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 and transmission risk
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
Posologia
Oral Zovirax Tablets
| Indicazione | Dose | Durata |
|---|---|---|
| First episode genital herpes | 200 mg five times daily (every 4 hours, omitting night dose) | 5–10 days |
| Recurrent genital herpes (episodic) | 200 mg five times daily | 5 days |
| Suppressive therapy | 400 mg twice daily | Ongoing (review annually) |
| Suppressive (in immunocompromised) | 400 mg three to five times daily | Specialist guidance |
| Cold sores (oral) | 200 mg five times daily | 5 days |
| Shingles | 800 mg five times daily | 7 days |
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.
Effetti collaterali
Oral aciclovir at standard doses is very well tolerated.
Effetti collaterali comuni (at standard doses)
- Nausea
- Headache
- Dizziness
- Fatigue
- Diarrhoea
Effetti collaterali non comuni
- 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
| Medicine | Oral Bioavailability | Dosing (episodic) | Dosing (suppressive) | Notes |
|---|---|---|---|---|
| Zovirax (Aciclovir) | 15–30% | 200 mg 5× daily (5 days) | 400 mg twice daily | Original agent; low bioavailability; lower cost |
| Valtrex (Valacyclovir) | ~55% (as aciclovir) | 500 mg twice daily (3–5 days) | 500 mg once daily | Prodrug; convenient; preferred clinically |
| Famvir (Famciclovir) | ~77% (as penciclovir) | 125 mg twice daily (5 days) | 250 mg twice daily | Different active metabolite; similar efficacy |
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).
How to access Zovirax online in Europe
Oral Zovirax (aciclovir tablets) is a prescription-only medicine (POM) in most EU member states. Non può essere legalmente dispensato senza una prescrizione valida da parte di un professionista medico autorizzato.
Prescrivia opera come intermediario tecnologico: non prescriviamo farmaci, non impieghiamo medici né vendiamo medicinali. La nostra piattaforma mette in contatto i pazienti con medici indipendenti registrati nell’UE che possono condurre valutazioni sanitarie online riservate.
Il processo:
- Completi una valutazione della salute: Describe your symptoms, history of herpes episodes, relevant medical history (especially kidney function), and any current medications.
- Revisione medica: Un medico indipendente registrato nell’UE esamina la Sua valutazione. If aciclovir is clinically appropriate, they may issue a prescription.
- Dispensazione in farmacia: La prescrizione viene inviata a una farmacia partner autorizzata dell’UE per la dispensazione e la consegna diretta.
Importante: Prescrivia non garantisce che venga emessa una prescrizione. Tutte le decisioni prescrittive vengono prese in modo indipendente da professionisti medici qualificati.
Informazioni importanti sulla sicurezza
Informi il Suo medico prima di taking Zovirax se Lei:
- 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 (probenecid, ciclosporin) — these may affect 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: Suppressive therapy and consistent condom use together provide the best available reduction in herpes transmission risk. 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.
Fonti
Le informazioni mediche presenti in questa pagina si basano sulle seguenti fonti:
- European Medicines Agency (EMA). Zovirax (aciclovir) — Summary of Product Characteristics. Disponibile su: ema.europa.eu
- 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.
Questo contenuto viene rivisto periodicamente per riflettere le linee guida cliniche aggiornate. Viene fornito esclusivamente a scopo informativo e non costituisce consulenza medica. Consulti sempre un professionista sanitario qualificato per consigli pertinenti alle Sue circostanze di salute individuali.
Frequently asked questions
- What is Zovirax used for?
- Zovirax (aciclovir) is an antiviral medicine approved by the EMA for the treatment of herpes simplex virus (HSV) infections — including genital herpes (HSV-1 and HSV-2) and cold sores (orolabial herpes) — and herpes zoster (shingles). It is available as oral tablets, topical cream, and intravenous formulation. In sexual health, oral aciclovir is used to treat genital herpes outbreaks and as suppressive therapy.
- How is Zovirax different from Valtrex?
- Zovirax (aciclovir) and Valtrex (valacyclovir) work through the same active molecule — aciclovir — and have equivalent clinical efficacy. The key difference is how they are absorbed: Valtrex is a prodrug that converts to aciclovir in the body, achieving blood levels 3–5 times higher than equivalent oral aciclovir doses. This higher bioavailability means Valtrex can be taken less frequently (typically twice daily) compared to Zovirax (five times daily for episodic treatment). Both are effective; the choice is often based on dosing convenience and cost.
- How long does a Zovirax course last for genital herpes?
- For a first episode of genital herpes, oral aciclovir is taken five times daily (every 4 hours during waking hours) for 5 to 10 days. For recurrent episodes treated at the prodrome (early warning signs), a 5-day course is typical. For suppressive therapy to reduce frequency of outbreaks, aciclovir 400 mg is taken twice daily continuously.
- Is Zovirax cream effective for genital herpes?
- Zovirax topical cream (5% aciclovir) is licensed for cold sores (orolabial herpes) and is widely used. However, for genital herpes, oral aciclovir (tablets) is substantially more effective than topical cream alone, as it achieves systemic concentrations capable of treating the infection from within. Oral treatment is the recommended approach for genital herpes.
- Can I get Zovirax online in Europe?
- Oral Zovirax (aciclovir tablets) is a prescription-only medicine in most EU member states. È possibile accedere a it through online intermediary platforms that connect you with independent EU-registered doctors for a confidential assessment and, if clinically appropriate, a prescription. Some topical formulations may be available without prescription for cold sores.
Trattamenti
Prescrivia è solo una piattaforma di intermediazione. Non forniamo servizi medici, non prescriviamo trattamenti e non dispensiamo farmaci. Tutte le decisioni mediche sono prese da medici indipendenti registrati nell'UE. Tutti i farmaci sono dispensati da farmacie autorizzate dell'UE. Questa piattaforma facilita i collegamenti tra pazienti e operatori sanitari.