An antiviral for CMV and resistant herpes
Foscarnet
A hospital drip used for CMV and resistant herpes infections when other antivirals fail, with careful kidney monitoring.
What is Foscarnet?
Foscarnet is a specialist antiviral given as a drip into a vein in hospital. It is used for cytomegalovirus (CMV) infection and for herpes infections that have become resistant to the usual antivirals, often when other treatments have not worked. Its main risks are harm to the kidneys and upset of the body's minerals, such as calcium, so plenty of fluids are given and frequent blood tests are needed. It is always given and monitored by a hospital team. The brand name is Foscavir.
Education and reference only. This is a plain-language guide to Foscarnet — it deliberately contains no doses. Doses depend on the person, the brand and the reason for treatment, and belong with your prescriber. Always check the BNF, the product labelling (SmPC) and follow medical advice.
What it is
Foscarnet is an antiviral medicine used to treat serious viral infections in people whose immune system is weakened. It is mainly used for cytomegalovirus (CMV) infection and for herpes simplex or shingles infections that have become resistant to the usual antivirals such as aciclovir or ganciclovir. It is given only as a drip into a vein, in hospital, because it needs careful fluid support and close monitoring. The brand name is Foscavir.
How it works
Foscarnet works differently from many antivirals: it directly blocks the part of the virus's copying enzyme that builds new genetic material, without needing to be activated by the virus first. This is why it can still work against viruses that have become resistant to other antivirals. Because it binds minerals such as calcium and is cleared by the kidneys, it can disturb the body's mineral balance and strain the kidneys, which shapes how it is given and monitored.
Company & origin
Originated / developed by: Generic (specialist).
A specialist antiviral given by drip in UK hospitals for CMV and resistant herpes infections when other treatments have not worked.
Practical use
How to take Foscarnet
General, dose-free guidance — always follow your prescriber's and the leaflet's specific instructions.
- It is given only as a drip into a vein by a hospital team, at a carefully controlled rate.
- Extra fluids are given before and during treatment to help protect your kidneys.
- Attend all your blood tests, which check your kidneys and your mineral levels frequently.
- Keep the genital area clean and rinse after passing urine, as the drug in the urine can irritate the skin.
- Report any tingling around the mouth or in the hands, muscle twitching, cramps or reduced urine output straight away.
Weighing it up
Advantages & disadvantages of Foscarnet
Advantages
- Can work against CMV and herpes infections that have become resistant to other antivirals.
- Acts directly on the virus's copying enzyme, offering an option when first-choice drugs fail.
- Given and closely monitored in hospital, so problems can be managed quickly.
Disadvantages
- Can harm the kidneys, so plenty of fluids and frequent monitoring are needed.
- Can disturb minerals such as calcium and magnesium, causing tingling, cramps or, rarely, fits.
- Can only be given as a hospital drip, not at home.
Practical use
Good to know
The most important cautions with foscarnet are its effects on the kidneys and on the body's minerals. It can reduce kidney function, so it is given with plenty of fluids into a vein and the kidneys are checked frequently. It can also disturb levels of calcium, magnesium, phosphate and potassium, which can cause tingling around the mouth or in the hands, muscle twitching, cramps or, rarely, fits, so minerals are monitored and corrected. Because of these risks it is given only in hospital, with a controlled rate of infusion and close nursing care. It can also cause irritation or ulcers in the genital area, so good hygiene after passing urine is advised. Tell the team straight away about any tingling, twitching, cramps or reduced urine output.
Who should not take it / use with caution
- People with significant kidney problems need careful dose adjustment and may not be suitable.
- Used with great caution in those with disturbances of calcium or other minerals.
- It is avoided in pregnancy unless the benefit clearly outweighs the risk, under specialist guidance.
Monitoring
- Frequent kidney function tests, with fluids and dose adjusted accordingly.
- Regular checks of calcium, magnesium, phosphate and potassium, correcting them as needed.
- Watching the response of the infection and reviewing for side effects throughout treatment.
Side effects
- Reduced kidney function and disturbances of calcium, magnesium, phosphate or potassium are common.
- Tingling around the mouth or in the hands, muscle cramps, nausea, vomiting and fever.
- Irritation or ulcers in the genital area, and rarely seizures with severe mineral changes.
Key interactions
- Combining it with other medicines that can harm the kidneys increases the risk to kidney function.
- Medicines or conditions that affect calcium and other minerals can add to its mineral disturbances.
- It can interact with certain other treatments, so your team reviews your full medicine list.
Available as: A drip into a vein, given in hospital.
Answers
Foscarnet: frequently asked questions
What is foscarnet used for?
It is a specialist antiviral for cytomegalovirus (CMV) infection and for herpes infections that have become resistant to the usual antivirals, often when other treatments have not worked.
Why do I need so many fluids and blood tests?
Foscarnet can affect the kidneys and the body's minerals, so extra fluids and frequent blood tests help protect the kidneys and keep minerals in balance.
Why do I feel tingling or cramps?
These can be signs that minerals such as calcium have been disturbed; tell the team straight away so the levels can be checked and corrected.
Can it be given at home?
No, it is given only as a drip in hospital because it needs careful fluid support and close monitoring.
Why is genital hygiene important during treatment?
The drug passed in the urine can irritate the skin and cause ulcers in the genital area, so rinsing and keeping the area clean after passing urine helps.
The wider class
About Antiviral (CMV / resistant herpes)
Foscarnet belongs to the antiviral (cmv / resistant herpes) class. For how the class as a whole works, its shared safety principles and monitoring, see the full guide.
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Authoritative sources
- BNF
- NICE CKS
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