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[Pharmacology](https://durgical.com/en/pharmacology/) · [Anti-herpesvirus agents](https://durgical.com/en/pharmacology/classes/anti-herpesvirus-agents/)

# Famciclovir

An oral prodrug of penciclovir for shingles and herpes simplex. Its active form stays inside infected cells for many hours.

Updated 2026-11-01

- Class: [Anti-herpesvirus agents](https://durgical.com/en/pharmacology/classes/anti-herpesvirus-agents/)
- Enzyme: Viral thymidine kinase (activation) and viral DNA polymerase
- ATC code: J05AB09

✂**Viral thymidine kinase (activation) and viral DNA polymerase**Famciclovir

ⓘ The figure shows penciclovir, the active form of famciclovir, in herpes simplex virus 1 thymidine kinase, the enzyme that starts its activation.

**Target in 3D:** Viral thymidine kinase (activation) and viral DNA polymerase. PDB structure 1KI3 (RCSB PDB, public domain). [View the structure at RCSB PDB](https://www.rcsb.org/structure/1KI3) · Drag to rotate · pinch to zoom

## Mechanism of action

Famciclovir is absorbed and converted to penciclovir, a guanosine analogue. Like aciclovir, penciclovir is phosphorylated first by the viral thymidine kinase and then by cellular enzymes. Penciclovir triphosphate inhibits the viral DNA polymerase and slows DNA elongation; it is weaker than aciclovir triphosphate but reaches higher levels and lasts much longer inside infected cells. Thymidine kinase-deficient, aciclovir-resistant strains are cross-resistant.

- Absorbed and converted to penciclovir
- Viral thymidine kinase phosphorylates penciclovir
- Penciclovir triphosphate inhibits the viral DNA polymerase
- It stays in infected cells for hours, prolonging the effect

## Pharmacokinetics

**Absorption**
Well absorbed by mouth (about 75%), unlike penciclovir itself

**Metabolism**
Converted to penciclovir during and after absorption

**Elimination**
Penciclovir mostly unchanged in the urine

**Half-life**
Penciclovir about 2 hours in plasma; its triphosphate lasts 7 to 20 hours inside infected cells

## Indications

- Herpes zoster (shingles) [Herpes zoster (shingles) →](https://durgical.com/en/conditions/herpes-zoster/)
- Recurrent genital herpes, and suppression of recurrences; recurrent herpes simplex in people with HIV [Genital herpes (herpes simplex infection) →](https://durgical.com/en/conditions/genital-herpes/)
- Recurrent cold sores in people with a normal immune system

## Adverse effects

### Common

- Headache
- Nausea, vomiting, diarrhoea and abdominal pain
- Tiredness
- Rash and itching

### Serious (rare, important)

- Confusion or disorientation
- Pain, burning, numbness or tingling in the hands or feet
- Severe allergic reactions

## Contraindications

- Hypersensitivity to famciclovir or penciclovir

## Key interactions

**Drugs cleared by the kidneys**
Kidney function, rather than drug interactions, is the main factor for penciclovir levels

## What is monitored

- Kidney function, because penciclovir is cleared by the kidneys
- Mental status in older people

## Administration and safety

- Oral, with or without food
- Started as early as possible in an outbreak or in shingles

## Nursing care

- Ask about kidney disease before starting
- Warn about possible drowsiness, dizziness or confusion before driving
- Teach to report numbness, burning or tingling in the hands or feet
- Explain that it does not cure herpes and may not prevent spread

## Pregnancy and breastfeeding

Human data are limited; the benefit and risk are weighed, as with other antivirals for herpes.

## Memory aid

FAM-ciclovir brings the PENciclovir «pen» that keeps writing inside the cell for hours.

## Test yourself

Think of the answer, then tap the question to see it.

## What is the active form of famciclovir?

Penciclovir, which is then phosphorylated to penciclovir triphosphate.

## Does famciclovir work against thymidine kinase-deficient, aciclovir-resistant herpes simplex?

No. Those strains are cross-resistant because penciclovir also needs the viral thymidine kinase.

## Why can famciclovir be given less often than aciclovir?

It is well absorbed and penciclovir triphosphate persists for many hours inside infected cells.

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## Related drugs

- [Aciclovir](https://durgical.com/en/pharmacology/aciclovir/)
- [Valaciclovir](https://durgical.com/en/pharmacology/valaciclovir/)

## Related conditions

- [Herpes zoster (shingles)](https://durgical.com/en/conditions/herpes-zoster/)
- [Genital herpes (herpes simplex infection)](https://durgical.com/en/conditions/genital-herpes/)

[← Back to the class: Anti-herpesvirus agents](https://durgical.com/en/pharmacology/classes/anti-herpesvirus-agents/)

Educational content on how drugs work; not a prescribing guide and no substitute for a health professional's advice. No doses are given.

Sources: [Famciclovir: MedlinePlus Drug Information](https://medlineplus.gov/druginfo/meds/a694038.html) (MedlinePlus (U.S. National Library of Medicine), 2026); [Herpes simplex virus](https://www.who.int/news-room/fact-sheets/detail/herpes-simplex-virus) (World Health Organization, 2025); [ATCDDD - ATC/DDD Index](https://atcddd.fhi.no/atc_ddd_index/?code=J05AB09) (WHO Collaborating Centre for Drug Statistics Methodology, 2026); Goodman & Gilman's The Pharmacological Basis of Therapeutics, 14th ed., ch. 62 (McGraw Hill, 2023).

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Representative model, for educational purposes only.

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