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

- Class: [Anti-herpesvirus agents](https://durgical.com/en/pharmacology/classes/anti-herpesvirus-agents/)
- Enzyme: Viral DNA polymerase (pyrophosphate-binding site)
- ATC code: J05AD01

✂**Viral DNA polymerase (pyrophosphate-binding site)**Foscarnet

ⓘ 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](https://www.rcsb.org/structure/8EXX) · Drag to rotate · pinch to zoom

## 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.

- Enters cells without needing activation
- Binds the pyrophosphate site of the viral DNA polymerase
- The polymerase cannot complete each nucleotide addition
- Viral DNA synthesis stops

## Pharmacokinetics

**Absorption**
Intravenous only

**Distribution**
Sequestered in bone and released slowly

**Elimination**
Mostly unchanged in the urine

## Indications

- CMV retinitis in people with AIDS, alone or with ganciclovir [Cytomegalovirus infection →](https://durgical.com/en/conditions/cytomegalovirus-infection/)
- Aciclovir-resistant mucocutaneous herpes simplex in immunocompromised people [Genital herpes (herpes simplex infection) →](https://durgical.com/en/conditions/genital-herpes/)

## Adverse effects

### Common

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

### Serious (rare, important)

- Kidney injury (the main toxicity)
- Low ionized calcium, magnesium and potassium, high or low phosphate
- Seizures related to these mineral changes
- QT prolongation and torsade de pointes

## 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.

## Memory aid

FOSCARnet «scars» Kidneys and Calcium: check K (kidney) and Ca before every dose.

## 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.

[Add to my cards (3)](https://durgical.app/estudiar/?add=/en/pharmacology/foscarnet/)

## Related drugs

- [Ganciclovir](https://durgical.com/en/pharmacology/ganciclovir/)
- [Cidofovir](https://durgical.com/en/pharmacology/cidofovir/)
- [Aciclovir](https://durgical.com/en/pharmacology/aciclovir/)

## Related conditions

- [Cytomegalovirus infection](https://durgical.com/en/conditions/cytomegalovirus-infection/)
- [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: [DailyMed - FOSCARNET injection, solution](https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=335f7deb-a045-43cb-b319-d7b73bfda73c) (DailyMed (U.S. National Library of Medicine), 2026); [Foscarnet Injection: MedlinePlus Drug Information](https://medlineplus.gov/druginfo/meds/a601144.html) (MedlinePlus (U.S. National Library of Medicine), 2026); [ATCDDD - ATC/DDD Index](https://atcddd.fhi.no/atc_ddd_index/?code=J05AD01) (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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Source: https://durgical.com/en/pharmacology/foscarnet/
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