Pharmacology · 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
The figure shows penciclovir, the active form of famciclovir, in herpes simplex virus 1 thymidine kinase, the enzyme that starts its activation.
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) →
- Recurrent genital herpes, and suppression of recurrences; recurrent herpes simplex in people with HIV Genital herpes (herpes simplex infection) →
- Recurrent cold sores in people with a normal immune system
Adverse effects
Common
- Headache
- Nausea, vomiting, diarrhoea and abdominal pain
- Tiredness
- Rash and itching
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.
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.
Related drugs
Related conditions
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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 (MedlinePlus (U.S. National Library of Medicine), 2026); Herpes simplex virus (World Health Organization, 2025); ATCDDD - ATC/DDD Index (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.