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Pharmacology · Anti-herpesvirus agents

Foscarnet !

An intravenous antiviral for resistant herpes simplex and CMV. It blocks the viral polymerase directly but harms the kidneys and lowers calcium.

Updated 2026-11-01

The figure shows foscarnet bound to the herpes simplex virus 1 DNA polymerase with DNA (cryo-EM).

Target in 3D: Viral DNA polymerase (pyrophosphate-binding site). PDB structure 8EXX (RCSB PDB, public domain). View the structure at RCSB PDB

Mechanism of action

Foscarnet is an analogue of pyrophosphate. It binds the pyrophosphate site of the viral DNA polymerase and stops it from releasing pyrophosphate after each nucleotide is added, so the chain cannot grow. It needs no phosphorylation, so it acts on thymidine kinase-deficient herpes simplex and UL97-mutant CMV.

  1. Enters cells without needing activation
  2. Binds the pyrophosphate site of the viral DNA polymerase
  3. The polymerase cannot complete each nucleotide addition
  4. Viral DNA synthesis stops

Pharmacokinetics

Absorption
Intravenous only
Distribution
Sequestered in bone and released slowly
Elimination
Mostly unchanged in the urine

Indications

Adverse effects

Common

  • Nausea, vomiting and headache
  • Fever and anaemia
  • Genital irritation and ulcers
  • Injection site reactions

Contraindications

  • Clinically significant hypersensitivity to foscarnet

Key interactions

Intravenous pentamidine
Symptomatic hypocalcaemia and added kidney toxicity
Other nephrotoxic drugs (aminoglycosides, amphotericin B, ciclosporin)
Higher risk of kidney injury
Drugs that prolong the QT interval
Higher risk of arrhythmia

What is monitored

  • Serum creatinine often
  • Calcium (ideally ionized), magnesium, potassium and phosphate
  • ECG when there are other QT risks
  • Eye examinations in CMV retinitis

Administration and safety

  • Given with an infusion pump, never as a rapid injection
  • Hydration with each course to protect the kidney
  • Diluted before infusion through a peripheral vein
  • High doses and fast infusion add to kidney and mineral toxicity

Nursing care

  • Ask about tingling around the mouth or in the fingers, muscle cramps or twitching (low calcium)
  • Have seizure precautions in place
  • Keep fluid balance and creatinine under close watch
  • Teach genital hygiene to reduce irritation and ulcers from drug in the urine

Pregnancy and breastfeeding

Human data are limited; used in pregnancy only if clearly needed.

Test yourself

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

Why does foscarnet work against aciclovir-resistant herpes simplex?

It binds the viral polymerase directly and needs no activation by the viral thymidine kinase.

Which mineral change explains tingling around the mouth during foscarnet infusion?

A fall in ionized calcium (foscarnet binds calcium); low magnesium can add to it.

Which intravenous drug greatly increases the risk of hypocalcaemia with foscarnet?

Pentamidine.

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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: DailyMed - FOSCARNET injection, solution (DailyMed (U.S. National Library of Medicine), 2026); Foscarnet Injection: MedlinePlus Drug Information (MedlinePlus (U.S. National Library of Medicine), 2026); 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.