ZOVIRAX 800MG TAB

Indications

Approved Uses

Treatment of herpes zoster (shingles), treatment and suppression of genital herpes simplex (HSV-2), treatment of herpes simplex labialis (cold sores), treatment of varicella (chickenpox) in immunocompromised patients

Off-Label Uses

Off-label/less common uses include HSV prophylaxis in immunocompromised patients (some indications may be labeled depending on jurisdiction), adjunct in Bell’s palsy with corticosteroids (evidence mixed), and limited/adjunctive use for EBV-related disease; CMV prophylaxis/treatment is generally not preferred (ganciclovir/valganciclovir are standard) though high-dose acyclovir has been studied historically.

Dosage & Administration

Dosing by Condition

Herpes zoster: 800 mg PO 5 times daily (about every 4 hours while awake) for 7-10 days. Genital herpes (initial episode): 400 mg PO 3 times daily for 7-10 days (alternative: 200 mg 5 times daily for 7-10 days). Genital herpes (recurrent): 800 mg PO twice daily for 5 days (alternatives exist). Genital herpes (suppression): 400 mg PO twice daily. Varicella: 800 mg PO 4 times daily for 5 days (typically start within 24 hours of rash).

Initial Dose

800 mg every 4 hours, 5 times a day, for 7 to 10 days for Herpes Zoster (shingles).

Maintenance Dose

For chronic suppression of genital herpes: 200 mg 3 times daily or 400 mg twice daily.

Maximum Dose

4000mg per day (800mg, 5 times daily).

Children's Dosage

Varicella in immunocompromised children ≥2 years: 20mg/kg (max 800mg) 4 times daily for 5 days. Herpes simplex in neonates and children: IV formulation preferred; consult specialist for weight-based dosing

Dose Adjustment Notes

Dose and/or dosing interval adjustment is required in renal impairment; ensure adequate hydration and use caution with concomitant nephrotoxic drugs; start therapy as early as possible in the course of infection for best benefit.

How to Take

Swallow the 800 mg tablet whole with water; take doses at evenly spaced intervals as prescribed and maintain adequate hydration; may be taken with or without food.

Side Effects

Common Side Effects

Nausea, vomiting, diarrhea, headache, malaise, dizziness, fatigue, rash, abdominal pain.

Side Effect Frequency

Common (≥1%): nausea, diarrhea, headache, vomiting, rash; less common/uncommon: dizziness, abdominal pain; rare/very rare: renal impairment/failure (especially with dehydration/renal disease), neurotoxicity (confusion, hallucinations, seizures), severe hypersensitivity including anaphylaxis, and severe cutaneous reactions (e.g., SJS/TEN).

Safety & Warnings

Contraindications

Hypersensitivity to acyclovir, valacyclovir, or any component of the formulation.

Warnings & Precautions

Ensure adequate hydration; adjust dose in renal impairment and use caution in the elderly and with nephrotoxic drugs; monitor renal function in high-risk patients; watch for neurotoxicity (especially with renal dysfunction); does not eradicate latent virus or fully prevent transmission; rare TTP/HUS reported in severely immunocompromised patients.

Age Restriction

Oral acyclovir 800 mg tablets are used in adults; pediatric use is indication- and weight-based and is generally for children ≥2 years for varicella (younger children/infants typically require alternative dosing/forms; neonatal disease requires IV acyclovir).

Drug Interactions

Drug Interactions

Key interactions: probenecid and cimetidine (↓ renal clearance → ↑ acyclovir exposure), mycophenolate mofetil (↑ levels of both), and other nephrotoxic drugs (additive kidney injury risk); zidovudine may increase CNS adverse effects in some patients; tizanidine is not a clinically established interaction.

Interaction Severity

MAJOR/clinically significant: concomitant nephrotoxic agents (e.g., aminoglycosides, amphotericin B, cyclosporine/tacrolimus, high-dose NSAIDs) increase risk of renal injury. MODERATE: probenecid and cimetidine can increase acyclovir exposure by reducing renal tubular secretion; mycophenolate mofetil may increase levels of both; zidovudine may increase CNS adverse effects. MINOR: antacids have no clinically meaningful effect.

Food Interaction

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

Special Populations

Children

Varicella in immunocompromised children ≥2 years: 20mg/kg (max 800mg) 4 times daily for 5 days. Herpes simplex in neonates and children: IV formulation preferred; consult specialist for weight-based dosing

Kidney Impairment

For the 800 mg regimen (e.g., zoster): CrCl 10-25 mL/min: 800 mg every 8 hours; CrCl <10 mL/min: 800 mg every 12 hours; in hemodialysis, give the dose after dialysis.

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

Complete the prescribed course; do not stop early, and do not discontinue long-term suppressive therapy without clinician advice (recurrences may return).

Overdose

Symptoms may include GI upset, agitation/confusion/hallucinations, seizures, lethargy/coma, and acute kidney injury with elevated creatinine/oliguria; management is supportive with aggressive hydration and hemodialysis can substantially remove acyclovir-seek urgent medical care.

Patient Counseling

Start as soon as symptoms appear; take exactly as prescribed at evenly spaced intervals and complete the full course; drink plenty of fluids; may take with or without food; this treats outbreaks but does not eradicate the virus or fully prevent transmission-avoid sexual contact during outbreaks and use barrier protection; seek care for reduced urination, severe rash, or CNS symptoms (confusion/hallucinations), especially if kidney disease or dehydration.

Monitoring Requirements

Monitor renal function (SCr/CrCl; consider BUN) in patients with renal impairment, older adults, high-dose therapy, dehydration, or concomitant nephrotoxins; monitor for neurotoxicity (confusion, hallucinations, tremor) especially with renal dysfunction.

Pharmacology

Mechanism of Action

Acyclovir is phosphorylated by viral thymidine kinase to the monophosphate and then by host kinases to acyclovir triphosphate, which competitively inhibits viral DNA polymerase and is incorporated into viral DNA causing chain termination.

Duration of Effect

Approximately 6-8 hours per dose (clinical antiviral effect maintained with repeated dosing; dosing commonly 4-5 times/day in normal renal function).

Half-Life

Elimination half-life ~2.5-3.3 hours in adults with normal renal function; prolonged to ~19-20 hours in end-stage renal disease (ESRD).

Bioavailability

Oral bioavailability ~10-20% (decreases with increasing dose).

Metabolism

Minimal hepatic metabolism; small fraction metabolized to 9-carboxymethoxymethylguanine (CMMG); pharmacologic activation occurs intracellularly via phosphorylation to acyclovir triphosphate.

Excretion

Primarily renal excretion; ~60-90% excreted unchanged in urine via glomerular filtration and active tubular secretion.

Protein Binding

9-33%

Product Information

Available Dosage Forms

Acyclovir is available as oral tablets, oral suspension, IV infusion/injection (powder/solution), topical cream/ointment, and ophthalmic ointment; capsule is not a standard acyclovir dosage form in most references.

Composition per Dose

Each tablet: 800mg acyclovir

Generic Availability

Yes

OTC Alternatives

No OTC alternative provides equivalent systemic therapy; for herpes labialis only, OTC options in some countries include docosanol 10% cream, and in some jurisdictions topical acyclovir may be non-prescription.

Viral Target

Herpes

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