Acyclovir Tablets – Oral Antiviral for Herpes Zoster, Genital Herpes, and Chickenpox

Acyclovir Tablets are an oral antiviral medication used to treat infections caused by herpesviruses. They are specifically indicated for managing shingles (herpes zoster), initial and recurrent genital herpes, and chickenpox (varicella).

Indications for Acyclovir Tablets include:

• Acute treatment of herpes zoster (shingles) in immunocompetent adults.
• Treatment of initial episodes and management of recurrent genital herpes infections.
• Treatment of chickenpox (varicella) in pediatric patients aged 2 years and older.

Treatment should be initiated promptly, preferably within 72 hours of symptom onset for shingles and within 24 hours of rash onset for chickenpox. The safety and efficacy in children under 2 years have not been established.

Acyclovir

Acyclovir is a synthetic purine nucleoside analogue that targets herpes simplex virus types 1 and 2 (HSV-1 and HSV-2) and varicella-zoster virus (VZV). It becomes activated through phosphorylation by the viral enzyme thymidine kinase (TK), converting it to acyclovir monophosphate. Cellular enzymes further convert it to acyclovir triphosphate, which inhibits viral DNA replication by:

• Competing with natural nucleotides for viral DNA polymerase;
• Being incorporated into and terminating the growing viral DNA chain;
• Inactivating viral DNA polymerase.

Acyclovir's greater activity against HSV is due to more efficient phosphorylation by viral TK compared to VZV.

General dosing notes: Acyclovir Tablets can be taken with or without food. Adequate hydration should be maintained to reduce renal toxicity risks. Dosage adjustment is required in patients with renal impairment.

Herpes Zoster (Shingles): 800 mg orally five times daily for 7 to 10 days. Treatment should start within 72 hours of rash onset for optimal effectiveness, particularly within the first 48 hours. Adults over 50 years show greater benefit.

Genital Herpes:

• Initial episode: 400 mg orally three times daily for 7 to 10 days.
• Recurrent episodes: 400 mg orally three times daily for 5 days or 800 mg twice daily for 5 days.
• Suppressive therapy for frequent recurrences (≥6 episodes/year): 400 mg orally twice daily for up to 10 years.

Chickenpox (Varicella) in pediatrics (aged 2 to 18 years): 20 mg/kg (up to 800 mg) orally four times daily for 5 days, started within 24 hours of rash onset.

Acyclovir is contraindicated in patients with known hypersensitivity to acyclovir or valacyclovir.

Important safety information:

• Acyclovir is contraindicated in patients with hypersensitivity to acyclovir or valacyclovir.
• Cases of renal failure, including deaths, have occurred, especially with improper hydration or in patients with underlying renal impairment.
• Thrombotic thrombocytopenic purpura/hemolytic uremic syndrome (TTP/HUS), sometimes fatal, has been reported in immunocompromised patients receiving acyclovir.
• Caution is advised when coadministering acyclovir with other nephrotoxic agents due to increased risk of renal dysfunction and reversible central nervous system effects.
• Avoid delayed treatment initiation in herpes zoster beyond 72 hours after rash onset, as efficacy is reduced.
• Acyclovir is not a cure for genital herpes and does not necessarily prevent transmission; patients should avoid contact with active lesions and practice safe behaviors.

Common side effects: Nausea, vomiting, and dizziness, especially in elderly patients.

Serious but rare adverse reactions: Renal failure, neurological symptoms such as somnolence, hallucinations, confusion, and coma, particularly in elderly or renally impaired patients. TTP/HUS has been reported in immunocompromised individuals.

Coadministration of probenecid with acyclovir can increase acyclovir plasma levels by reducing renal clearance, thereby increasing half-life and systemic exposure. Monitoring and potential dosage adjustments may be necessary.

Pregnancy: Acyclovir is classified as Pregnancy Category B. Animal studies have not shown teratogenic effects at doses producing plasma levels higher than in humans. Human data from a pregnancy registry including 749 first-trimester exposures show birth defect rates similar to the general population. Use during pregnancy should only occur if the potential benefit justifies potential fetal risk.

Lactation: Acyclovir is excreted in human breast milk at concentrations up to 4.1 times plasma levels, potentially exposing nursing infants to up to 0.3 mg/kg/day. Use with caution in breastfeeding women and only when necessary.

Overdose can cause renal failure, sometimes fatal. Management includes monitoring renal function and supportive care. Adequate hydration is critical to prevent crystal-induced nephropathy.