Penvir

✅ Treats Viral Infections
✅ Reduces Cold Sore Duration
✅ Alleviates Shingles Symptoms
✅ Prescribed Antiviral Medication
✅ Oral Medication Option

Penvir contains Famciclovir.

SKU: N/A Categories: Category Overview, Genital Herpes Treatment, Sexual Health Tags: Famciclovir, Penvir

✓ Medically reviewed by Morgan Ellis — Pharmacy Researcher · 8 years experience · Last reviewed: May 2026

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Strength

250 mg
500 mg

Quantity

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💡 Quick Answer

Penvir is famciclovir, a prescription antiviral prodrug of penciclovir for genital herpes (HSV-2), cold sores (HSV-1), and shingles (VZV). Penciclovir has an extended intracellular half-life (~10 hours) — so famciclovir dosing is more forgiving than aciclovir. Typical outpatient dosing is once or twice daily.

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What Is Penvir?

Penvir is a branded generic of famciclovir, an oral prodrug of penciclovir. Famciclovir has ~77% oral bioavailability (vs ~5% for oral penciclovir directly, and ~15–20% for oral aciclovir). Inside herpes-virus-infected cells, penciclovir triphosphate blocks viral DNA polymerase — the same final mechanism as aciclovir.

The clinical edge famciclovir offers is intracellular persistence: penciclovir triphosphate has a half-life of 10+ hours inside infected cells, vs ~1 hour for aciclovir triphosphate. This lets famciclovir use less-frequent dosing. Available in strengths of 250 mg, 500 mg.

Clinical Uses

Genital herpes (first episode): 250 mg three times daily for 7–10 days
Genital herpes (recurrent episodic): 1000 mg twice daily for one day, OR 125 mg twice daily for 5 days
Genital herpes (suppressive): 250 mg twice daily
Cold sores (herpes labialis): 1500 mg as a single dose, within 1 hour of prodrome
Shingles (herpes zoster): 500 mg three times daily for 7 days — start within 72 hours of rash onset
Recurrent HSV in HIV-positive patients: 500 mg twice daily for 7 days

How to Take

Swallow the tablet whole with a glass of water, with or without food.
Start at the first sign of an outbreak (tingling, burning, itching) — the single-dose cold-sore regimen only works if given within 1 hour of prodrome.
Stay well hydrated; famciclovir is renally cleared.
Complete the full course, even if symptoms resolve.

Side Effects

Common (≥1%): headache, nausea, diarrhoea, fatigue. Generally mild.

Less common: rash, dizziness.

Rare serious: transient elevations in liver enzymes, low platelets, acute kidney injury at very high doses in dehydrated patients.

Who Should Not Take Penvir

Known hypersensitivity to famciclovir or penciclovir
Dose reduction required for creatinine clearance <60 mL/min
Caution in pregnancy — less safety data than aciclovir; discuss with clinician
Hereditary galactose intolerance (some formulations contain lactose)

Drug Interactions

Probenecid, cimetidine: may increase penciclovir levels.
Aldehyde oxidase inhibitors (e.g. raloxifene): may reduce famciclovir conversion.
No significant interactions with hormonal contraceptives or antiretrovirals at standard doses.

Storage

Store at room temperature (15–30 °C / 59–86 °F), away from moisture and light. Keep in original blister. Keep out of reach of children.

Famciclovir vs Valacyclovir vs Aciclovir

All three are first-line HSV/VZV antivirals with comparable efficacy when dosed correctly. Famciclovir’s unique advantage is its long intracellular half-life, allowing single-dose cold sore treatment. Valacyclovir leads on convenience for daily suppression. Aciclovir remains the cost-leader. Choice usually comes down to dosing schedule, pricing, and clinician preference.

See the full comparison: Daily vs Episodic Herpes Treatment.

Frequently Asked Questions

Does Penvir cure herpes?

No. Penvir suppresses viral replication and shortens outbreaks but cannot eliminate the virus. Herpes remains latent in nerve ganglia for life.

Is the single-dose cold sore regimen really effective?

Yes, but timing is critical. 1500 mg as a single dose, taken within 1 hour of the prodromal tingle, reduces lesion duration and pain comparably to the 5-day aciclovir regimen. Miss that window and the benefit drops.

How does famciclovir differ from valacyclovir?

Both are prodrugs. Famciclovir converts to penciclovir; valacyclovir converts to aciclovir. The key practical difference is penciclovir’s longer intracellular half-life (~10 h) vs aciclovir’s (~1 h). Famciclovir thus supports less-frequent dosing and the single-dose cold sore regimen, which aciclovir/valacyclovir cannot match.

Can I take Penvir during pregnancy?

Less safety data than aciclovir or valacyclovir. Most clinicians prefer aciclovir/valacyclovir for pregnancy. Discuss with your obstetrician.

How quickly does Penvir work?

For episodic treatment, most patients feel improvement within 24–48 hours. Lesion healing is accelerated by 1–2 days vs untreated.

What if I miss a dose?

Take it as soon as you remember unless it is nearly time for the next dose. Never double-dose.

Can famciclovir be used for shingles?

Yes — 500 mg three times daily for 7 days. Start within 72 hours of rash onset for best outcome, especially to reduce post-herpetic neuralgia risk.

Any interactions with hormonal contraception?

No clinically significant interaction. Hormonal contraceptives remain effective.

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⚕️ Medical Disclaimer: Information on this page is for educational purposes only and does not replace medical advice. Always consult a qualified clinician before starting, stopping, or changing any medication. Prescription products should be used only under medical supervision.