A 45-year-old man with dermatomal vesicular rash and severe pain on the right thorax is diagnosed with herpes zoster. He presents on day 4 of illness. What is the drug of choice for treatment?

Explanation

## First-Line Antiviral for Herpes Zoster (Shingles) **Key Point:** Valacyclovir is the preferred first-line antiviral for herpes zoster (VZV) in immunocompetent adults due to superior oral bioavailability, less frequent dosing, and equivalent efficacy to intravenous acyclovir. ### Rationale for Valacyclovir Over Acyclovir **High-Yield:** For zoster, valacyclovir is preferred because: 1. **Oral bioavailability:** 54% vs acyclovir's 15–20% 2. **Dosing convenience:** 1000 mg three times daily vs acyclovir 800 mg five times daily 3. **Equivalent efficacy to IV acyclovir:** Achieves serum levels comparable to IV acyclovir 4. **Reduces postherpetic neuralgia (PHN)** when started within 72 hours 5. **Reduces viral dissemination and complications** ### Treatment Algorithm for Herpes Zoster ```mermaid flowchart TD A[Herpes Zoster Diagnosis]:::outcome --> B{Immunocompetent?}:::decision B -->|Yes| C{Within 72 hours?}:::decision B -->|No| D[IV Acyclovir 10-15 mg/kg/8h]:::action C -->|Yes| E[Valacyclovir 1000 mg TID × 7 days]:::action C -->|No| F[Consider treatment if severe pain/complications]:::action E --> G[Reduces PHN, viral dissemination]:::outcome D --> H[Immunocompromised protection]:::outcome ``` ### Comparative Dosing and Efficacy | Drug | Indication | Dosage | Bioavailability | Advantage | Limitation | |------|-----------|--------|-----------------|-----------|------------| | **Valacyclovir** | Zoster (immunocompetent) | 1000 mg TID × 7 days | 54% | First-line; convenient dosing | Cost | | **Acyclovir** | Zoster (if valacyclovir unavailable) | 800 mg 5× daily × 7 days | 15–20% | Cheap; long experience | Frequent dosing; lower bioavailability | | **Famciclovir** | Alternative for zoster | 500 mg TID × 7 days | 77% | Excellent bioavailability | Less data than valacyclovir in zoster | | **IV Acyclovir** | Zoster (immunocompromised) | 10–15 mg/kg IV 8-hourly | 100% (IV) | Essential for severe/disseminated | Requires hospitalization; nephrotoxicity risk | ### Timing and Efficacy **Clinical Pearl:** Antiviral therapy is most effective when started within **72 hours (3 days)** of rash onset. This patient presented on day 4, which is slightly beyond the ideal window, but treatment may still be considered if pain is severe or complications are present. **Warning:** Do NOT use acyclovir as first-line for zoster in immunocompetent adults — its poor oral bioavailability (15–20%) makes it suboptimal compared to valacyclovir or famciclovir. ### Additional Measures - Analgesics (NSAIDs, paracetamol, gabapentin for neuropathic pain) - Topical capsaicin or lidocaine patches - Consider tricyclic antidepressants (amitriptyline) if PHN develops