A 32-year-old woman presents with painful vesicular lesions on the lower lip and surrounding skin for 3 days. She has a history of recurrent herpes simplex labialis. What is the drug of choice for treatment?

Question

Accepted Answer

C. Acyclovir. ## First-Line Antiviral for Herpes Simplex Labialis

**Key Point:** Acyclovir is the gold-standard, first-line antiviral for herpes simplex virus (HSV) infection of the skin, including recurrent labial herpes.

### Mechanism of Action
Acyclovir is a nucleoside analogue that:
1. Undergoes phosphorylation by viral thymidine kinase (TK)
2. Forms acyclovir triphosphate
3. Inhibits viral DNA polymerase and causes chain termination
4. Achieves selective toxicity because viral TK phosphorylates it ~200× more efficiently than cellular kinases

### Dosing for Recurrent Labial Herpes
- **Topical:** 5% cream applied 5–6 times daily for 5–7 days (best if started within 48 hours of symptom onset)
- **Oral:** 400 mg 5 times daily for 5 days (for more severe or disseminated disease)

### Comparative Antiviral Efficacy

| Drug | Bioavailability | Dosing Frequency | Indication | Notes |
|------|-----------------|------------------|-----------|-------|
| Acyclovir | 15–20% (oral) | 5× daily | HSV-1/2, VZV (all types) | First-line; most experience |
| Valacyclovir | 54% (oral) | 2–3× daily | HSV-1/2, VZV | Prodrug of acyclovir; better oral absorption |
| Famciclovir | 77% (oral) | 2–3× daily | HSV-1/2, VZV | Prodrug of penciclovir; good for immunocompromised |
| Penciclovir | Limited (topical) | 2 hourly | Labial HSV only | Topical only; shorter duration of viral shedding |

**High-Yield:** Acyclovir remains the gold standard for initial and recurrent HSV skin infections due to:
- Decades of clinical experience and safety data
- Lowest cost
- Efficacy when started early (within 48–72 hours)
- Oral and topical formulations available

**Clinical Pearl:** Valacyclovir and famciclovir are prodrugs with superior oral bioavailability and less frequent dosing, making them preferred in immunocompromised patients or when compliance is a concern. However, for straightforward recurrent labial herpes in an immunocompetent patient, acyclovir remains first-line.

**Warning:** Penciclovir is topical-only and has no systemic bioavailability; it is not appropriate for systemic HSV infection or severe labial disease.

Answer

A. Penciclovir

Answer

B. Valacyclovir

Answer

D. Famciclovir

A 32-year-old woman presents with painful vesicular lesions on the lower lip and surrounding skin for 3 days. She has a history of recurrent herpes simplex labialis. What is the drug of choice for treatment?

Explanation

First-Line Antiviral for Herpes Simplex Labialis

Mechanism of Action

Dosing for Recurrent Labial Herpes

Comparative Antiviral Efficacy

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Drug	Bioavailability	Dosing Frequency	Indication	Notes
Acyclovir	15–20% (oral)	5× daily	HSV-1/2, VZV (all types)	First-line; most experience
Valacyclovir	54% (oral)	2–3× daily	HSV-1/2, VZV	Prodrug of acyclovir; better oral absorption
Famciclovir	77% (oral)	2–3× daily	HSV-1/2, VZV	Prodrug of penciclovir; good for immunocompromised
Penciclovir	Limited (topical)	2 hourly	Labial HSV only	Topical only; shorter duration of viral shedding