Acyclovir is a synthetic purine nucleoside analogue with potent inhibitory activity against human herpesviruses, primarily Herpes Simplex Virus (HSV) types 1 and 2, and Varicella-Zoster Virus (VZV). The 800mg tablet is a high-strength formulation primarily indicated for the suppression (prevention) of recurrent genital herpes and the treatment of herpes zoster (shingles). It acts as a prodrug, requiring viral thymidine kinase for activation, leading to selective inhibition of viral DNA synthesis.
Adult: Herpes Zoster: 800mg orally every 4 hours (5 times daily) for 7-10 days. Recurrent Genital Herpes Suppression: 800mg orally in divided doses (typically 400mg twice daily or 200mg 3-5 times daily). Chronic suppression: 400mg twice daily is standard; 800mg daily in divided doses may be used.
Note: Administer with or without food. Take with a full glass of water. Maintain adequate fluid intake to ensure good hydration and prevent renal toxicity. For suppression therapy, doses should be evenly spaced. Do not crush or chew unless advised (some brands are scored).
Acyclovir is a prodrug that requires three phosphorylation steps to become active. Viral thymidine kinase (TK), present in herpesvirus-infected cells, monophosphorylates acyclovir with high efficiency. Cellular kinases then convert it to the active diphosphate and triphosphate forms. Acyclovir triphosphate competitively inhibits viral DNA polymerase and, to a lesser extent, acts as a chain terminator when incorporated into the growing viral DNA chain.
Pregnancy: Pregnancy Category B (US FDA). Animal studies show no risk, but adequate human studies are lacking. Used for life-threatening maternal HSV infections (e.g., disseminated, encephalitis). For recurrent genital herpes suppression near term, it may be used to prevent neonatal herpes. Use only if clearly needed.
Driving: May cause dizziness, drowsiness, or confusion in some patients, especially with renal impairment. Patients should be cautioned about operating machinery or driving until they know how the drug affects them.
| Probenecid | Increases acyclovir plasma concentration and half-life by reducing renal tubular secretion. Risk of acyclovir toxicity. | Moderate |
| Other Nephrotoxic Drugs (Aminoglycosides, Amphotericin B, Ciclosporin, IV Pentamidine, Foscarnet) | Increased risk of renal dysfunction. Concurrent use requires intensive renal monitoring. | Major |
| Zidovudine | May potentiate neurotoxicity (lethargy, fatigue). | Moderate |
| Mycophenolate Mofetil | Potential for increased concentration of both drugs; monitor for hematological toxicity. | Moderate |