ZOVIRAX 400/MG TAB 70/TAB

Indications

Approved Uses

Treatment of herpes simplex infections (e.g., genital herpes-initial and recurrent; mucocutaneous HSV in immunocompromised patients), suppression (prophylaxis) of recurrent genital herpes, treatment of herpes zoster (shingles), and treatment of varicella (chickenpox).

Off-Label Uses

Off-label uses include HSV suppression/prophylaxis in certain high-risk settings (beyond labeled populations), adjunctive therapy for HSV keratitis (systemic), and limited/selected use in EBV-related conditions; CMV prophylaxis has historical use but is generally not preferred versus valganciclovir in modern transplant protocols.

Dosage & Administration

Dosing by Condition

Genital herpes (initial episode): 400 mg PO three times daily for 7-10 days (alternative: 200 mg five times daily). Recurrent genital herpes: 400 mg PO three times daily for 5 days (alternative: 200 mg five times daily). Suppressive therapy: 400 mg PO twice daily. Herpes zoster: 800 mg PO five times daily for 7-10 days. Varicella (adults): 800 mg PO four times daily for 5 days (start as early as possible).

Initial Dose

200mg every 4 hours 5 times daily (for initial genital herpes)

Maintenance Dose

400mg twice daily (for suppressive therapy of genital herpes)

Maximum Dose

4000mg per day (800mg 5 times daily for herpes zoster)

Children's Dosage

Varicella (children >2 years and <40kg): 20mg/kg (max 800mg) 4 times daily for 5 days. Herpes simplex (neonates and children <2 years): IV formulation preferred; oral dosing under specialist guidance. Herpes zoster in immunocompromised children: 250-500mg/m² IV every 8 hours

Dose Adjustment Notes

Dose reduction/interval extension is required in renal impairment based on creatinine clearance; maintain adequate hydration; no routine hepatic dose adjustment is required.

How to Take

Swallow tablets whole with a full glass of water; may be taken with or without food; maintain adequate hydration during therapy (especially with high doses or renal impairment).

Side Effects

Common Side Effects

Nausea, vomiting, diarrhea, abdominal pain, headache, dizziness, fatigue, rash, photosensitivity

Side Effect Frequency

Common (≥1%): nausea, vomiting, diarrhea, headache (and sometimes rash/dizziness); uncommon/rare: renal effects (increased creatinine, crystalluria/AKI), neurotoxicity (confusion, hallucinations, somnolence-more likely with renal impairment), hematologic abnormalities (e.g., thrombocytopenia/leukopenia), and very rare severe hypersensitivity/SCAR (e.g., SJS/TEN, anaphylaxis).

Safety & Warnings

Contraindications

Hypersensitivity to acyclovir, valacyclovir, or any excipients in the formulation.

Warnings & Precautions

Maintain adequate hydration; use caution and adjust dose in renal impairment and in the elderly; monitor renal function in high-risk patients and avoid/monitor with nephrotoxic co-medications; watch for neurotoxicity (especially with renal dysfunction); rare TTP/HUS risk in immunocompromised patients; acyclovir reduces symptoms/viral shedding but does not eradicate latent virus or fully prevent transmission.

Age Restriction

Oral acyclovir tablets can be used in adults and children; for varicella (chickenpox) treatment, oral acyclovir is generally used in children ≥2 years, while use in infants <2 years (and neonates) is not routinely established for tablets and should be specialist/physician-directed (often using alternative formulations/dosing).

Drug Interactions

Drug Interactions

Key interactions: probenecid and cimetidine can increase acyclovir exposure by reducing renal clearance; concomitant nephrotoxic agents (e.g., aminoglycosides, cyclosporine/tacrolimus, high-dose NSAIDs) increase risk of renal toxicity; mycophenolate mofetil may increase acyclovir (and MPAG) levels; zidovudine may increase CNS adverse effects (e.g., lethargy/somnolence).

Interaction Severity

MODERATE/CLINICALLY SIGNIFICANT: Probenecid and cimetidine can increase acyclovir exposure by reducing renal tubular secretion; additive nephrotoxicity risk with other nephrotoxic agents (e.g., aminoglycosides, cyclosporine/tacrolimus, amphotericin B, high-dose NSAIDs) warrants caution and renal monitoring; other interactions are generally minor/monitoring-based.

Food Interaction

No clinically significant food interaction; may be taken with or without food.

Special Populations

Children

Varicella (children >2 years and <40kg): 20mg/kg (max 800mg) 4 times daily for 5 days. Herpes simplex (neonates and children <2 years): IV formulation preferred; oral dosing under specialist guidance. Herpes zoster in immunocompromised children: 250-500mg/m² IV every 8 hours

Kidney Impairment

Dose adjustment is required in renal impairment; typical oral adjustments: for regimens using 400 mg q8h, reduce to 400 mg q12h when CrCl 10-25 mL/min and to 400 mg q24h when CrCl <10 mL/min; for regimens using 800 mg (e.g., zoster), reduce to 800 mg q12h when CrCl 10-25 mL/min and to 800 mg q24h when CrCl <10 mL/min; give doses after hemodialysis on dialysis days.

Storage & Patient Advice

Missed Dose

Take the missed dose as soon as remembered; if it is close to the next scheduled dose, skip the missed dose and resume the regular schedule; do not double doses.

Stopping the Medicine

For episodic treatment, complete the prescribed course; for chronic suppressive therapy, do not stop without clinician review because recurrences may return and the ongoing need should be reassessed periodically.

Overdose

Symptoms may include nausea/vomiting, headache, and prominent neurotoxicity (agitation, confusion, hallucinations, seizures, lethargy) and acute kidney injury/raised creatinine; management is urgent medical assessment, supportive care with hydration and renal monitoring, and hemodialysis can enhance removal in severe toxicity/renal failure.

Patient Counseling

Complete the full prescribed course; start as early as possible at first symptoms; maintain good hydration to reduce renal crystalluria/AKI risk; this medicine reduces symptoms/viral shedding but does not eradicate herpes and does not prevent transmission-use barrier protection/avoid sexual contact during outbreaks; inform the prescriber about kidney disease and other nephrotoxic drugs; seek urgent care for reduced urine output, severe rash, or neurotoxicity (confusion/hallucinations), and report unusual bruising/bleeding if it occurs.

Monitoring Requirements

Monitor renal function (SCr/CrCl) and ensure adequate hydration, especially with high doses, older adults, dehydration, concomitant nephrotoxins, or pre-existing renal impairment; consider CBC only in select immunocompromised/prolonged-therapy situations.

Pharmacology

Mechanism of Action

Acyclovir is a guanosine nucleoside analogue activated by viral thymidine kinase to the triphosphate form, which competitively inhibits viral DNA polymerase and is incorporated into viral DNA causing chain termination and inhibition of viral DNA synthesis.

Onset of Action

Pharmacologic antiviral activity begins after absorption (peak levels typically within ~1.5-2 hours orally), with clinical symptom improvement usually over 1-3 days when started early.

Duration of Effect

Short duration requiring frequent dosing: typical oral dosing intervals are every ~4 hours while awake (e.g., 5 times daily) or every 8-12 hours depending on indication and regimen.

Half-Life

About 2.5 to 3.3 hours in normal renal function; markedly prolonged in severe renal impairment/ESRD (up to ~19 hours).

Bioavailability

Approximately 10% to 20% oral bioavailability (often cited ~15% to 30% depending on dose).

Metabolism

Minimal hepatic metabolism; a small fraction is converted to the inactive metabolite 9-carboxymethoxymethylguanine (CMMG), while antiviral activity requires intracellular phosphorylation to acyclovir triphosphate by viral/cellular kinases.

Excretion

Primarily renal elimination via glomerular filtration and active tubular secretion, with most of the dose excreted unchanged in urine (commonly reported ~60% to 90% unchanged).

Protein Binding

Low protein binding, approximately 9% to 33% (commonly ~15%).

Product Information

Available Dosage Forms

Tablet (oral), oral suspension, powder for solution for IV infusion, topical cream/ointment, and ophthalmic ointment.

Composition per Dose

Each tablet: 400mg acyclovir

Generic Availability

Yes

OTC Alternatives

No OTC oral/systemic alternative to acyclovir; for OTC cold-sore (herpes labialis) options, docosanol 10% cream is OTC in many markets, while topical acyclovir 5% cream may be OTC in some countries but is not universally OTC and does not substitute for systemic therapy.

Viral Target

Herpes (HSV-1, HSV-2, VZV)

Reviewed by Al Mujtama Pharmacy Medical Review Team. Last reviewed 03-07-2026