Valacyclovir is an oral prodrug antiviral agent, rapidly and nearly completely converted to acyclovir and L-valine by first-pass intestinal and/or hepatic metabolism. Acyclovir is a synthetic purine nucleoside analogue with potent in vitro and in vivo inhibitory activity against human herpesviruses, including Herpes simplex virus types 1 (HSV-1) and 2 (HSV-2), Varicella-zoster virus (VZV), Epstein-Barr virus (EBV), and Cytomegalovirus (CMV). It is a mainstay of therapy for herpes zoster (shingles), herpes labialis (cold sores), and genital herpes in the Indian context.
Adult: Herpes Zoster: 1g (1000mg) three times daily for 7 days. Genital Herpes (Initial): 1g twice daily for 10 days. Recurrent Genital Herpes: 500mg twice daily for 3-5 days. Suppression of Genital Herpes: 500mg once daily (or 1g once daily for >10 recurrences/year). Herpes Labialis: 2g twice daily for 1 day (taken 12 hours apart).
Note: Can be taken with or without food. Swallow tablet whole with a full glass of water. Maintain adequate hydration. For episodic treatment, initiate at the earliest sign or symptom (e.g., prodrome).
Valacyclovir is converted to acyclovir, which is phosphorylated by viral thymidine kinase (TK) to acyclovir monophosphate. Cellular enzymes then convert it to acyclovir diphosphate and triphosphate. Acyclovir triphosphate competitively inhibits viral DNA polymerase and incorporates into the viral DNA chain, causing irreversible chain termination.
Pregnancy: Pregnancy Category B. Animal studies show no risk, but adequate human studies are lacking. Use only if clearly needed, weighing benefits against risks. Acyclovir (the active form) has been used in pregnancy for serious herpes infections.
Driving: May cause dizziness, somnolence, or confusion. Patients should not drive or operate machinery if they experience these effects.
| Probenecid | Decreases renal clearance of acyclovir, increasing plasma levels and risk of toxicity. | Moderate |
| Cimetidine | May reduce renal clearance of acyclovir, increasing AUC. | Mild |
| Mycophenolate Mofetil | Potential additive effect on immunosuppression; monitor for hematological toxicity. | Moderate |
| Nephrotoxic drugs (Aminoglycosides, NSAIDs, IV Ciclosporin, Tacrolimus) | Increased risk of renal dysfunction. Monitor renal function closely. | High |
| Zidovudine | Potential to cause drowsiness and lethargy. | Mild |