What is Foscavir: Uses, Warnings, Interactions & FAQs

Foscavir (foscarnet sodium) is a powerful antiviral agent most often reserved for high-risk or immunocompromised patients battling severe viral diseases like cytomegalovirus (CMV) and drug-resistant herpes simplex virus (HSV) infections. ¹

This blog will walk you through Foscavir’s uses, safety warnings, major interactions, and practical FAQs.

What Is Foscavir? 

Foscavir is an intravenous antiviral medication for viral strains that don’t have very effective first-line treatments.¹ It’s mainly used for:

    - CMV retinitis in AIDS patients: Foscarnet is FDA-approved to treat CMV and CMV-associated ophthalmic retinitis in individuals with AIDS who cannot tolerate     ganciclovir.¹

    - Drug-resistant CMV: It may be used as salvage therapy for patients with CMV infections in whom ganciclovir therapy has failed.¹

    - Acyclovir-resistant HSV: Foscarnet is approved for treating mucocutaneous herpes simplex virus (HSV) infections in immunocompromised patients who do not respond to acyclovir.¹

Note: Foscavir doesn’t work against HIV because it isn’t an antiretroviral drug.¹

How Does Foscavir Work?

Foscavir is a pyrophosphate analog, meaning it mimics a natural part of the DNA molecule. It works by attaching to the viral DNA polymerase enzyme and stopping the DNA chain from growing. Normally, the polymerase removes pyrophosphate to add new DNA pieces, but foscarnet blocks this step. It mainly targets viral enzymes, but in very high doses, it can also affect human DNA polymerase, though such doses are rarely needed. Resistance to foscarnet can happen due to mutations in the UL54 gene.¹

What are the side effects of Foscavir?

Side effects possible with Foscavir include:¹

    - Nausea and other gastrointestinal symptoms

    - Electrolyte imbalances (hypocalcemia, hypomagnesemia, hypokalemia)

    - Nephrotoxicity (acute kidney injury, tubular damage, crystal nephropathy)

    - Arginine vasopressin resistance (AVP-R) / nephrogenic diabetes insipidus

    - Seizures

    - Genital ulcers (more common in males)

    - Anemia

    - QT prolongation and torsades de pointes

Monitoring While on Foscavir

Given the various adverse effects associated with foscarnet administration, it is essential to monitor specific parameters regularly to ensure patient safety and treatment efficacy.¹

    - Check 24-hour creatinine clearance before starting and regularly during treatment to monitor renal function.

    - Obtain baseline and regular ECGs to assess for potential cardiac effects.

    - Measure baseline electrolytes—including calcium, potassium, sodium, magnesium, and phosphorus—and repeat weekly during maintenance therapy.

    - Monitor calcium ion status closely, especially early in treatment, to detect early signs of renal dysfunction.

    - Be alert for signs of metabolic disturbances, such as hypokalemic hyperchloremic metabolic acidosis.

    - Watch for clinical symptoms like muscle twitching or anxiety, which may indicate electrolyte imbalances needing more frequent testing.

    - Assess hydration status before infusion to reduce the risk of nausea and renal toxicity.

    - Conduct a complete blood count (CBC) to track hematocrit, hemoglobin, and white blood cell counts.

    - Monitor for hypersensitivity reactions throughout treatment to catch early signs of an allergic response.

    - Adjust dosing based on renal function and patient tolerance, using clinical judgment and available lab values to guide therapy.

Safety Warnings & Key Precautions for Foscavir

Warnings to be kept in mind when using Foscavir:¹

    - Foscarnet can cause serious kidney damage; regular monitoring of kidney function is essential.

    - The drug may lead to seizures due to changes in blood minerals like calcium, magnesium, and potassium.

    - It should only be used in immunocompromised patients with CMV retinitis or acyclovir-resistant HSV infections.

    - Foscarnet should not be used in patients with known hypersensitivity reactions to the drug, such as anaphylaxis or angioedema.

Here are the precautions you can take when on Foscavir: ¹

    - Foscarnet must be used cautiously in patients with abnormal EKGs due to chronic heart disease.

    - The drug should be avoided in individuals with chronic kidney disease or very low creatinine clearance (

    - Patients with electrolyte imbalances must have these corrected before starting foscarnet therapy.

    - Foscarnet should be used with caution in people with a history of seizures.

    - The drug is not recommended for use in patients undergoing hemodialysis.

    - Mild to moderate increases in liver enzymes (ALT) may occur and are usually reversible after stopping the drug.

    - Rare cases of cholestatic liver injury have been reported during foscarnet therapy.

    - In pregnancy, foscarnet should be used only if clearly needed, as animal studies show skeletal anomalies even at low doses.

    - It is not known if foscarnet passes into human breast milk, but high levels have been found in rat milk.

    - Women should consider discontinuing breastfeeding or the drug, especially since CDC advises HIV-positive mothers not to breastfeed.

    - Foscarnet can accumulate in the bones and teeth of children and may cause enamel defects.

    - The safety of foscarnet in pediatric patients has not been established and requires close monitoring of kidney and bone health.

    - In elderly patients, dose adjustments may be necessary due to age-related decline in kidney function.

Drug Interactions for Foscavir

Major drugs interactions possible with Foscavir include:¹

    - Cidofovir and foscarnet should not be used together due to the increased risk of additive nephrotoxicity, which can cause significant kidney damage.

    - Pimozide and thioridazine are contraindicated with foscarnet as the combination can worsen QT prolongation, raising the risk of serious heart arrhythmias.

    - Use of pentamidine with foscarnet requires caution because it can lead to hypocalcemia and kidney impairment.

    - Avoid combining foscarnet with nephrotoxic drugs like aminoglycosides, amphotericin B, cyclosporine, acyclovir, methotrexate, and tacrolimus unless absolutely necessary, as this increases the risk of renal toxicity.

    - Loop diuretics can interfere with foscarnet’s elimination and raise toxicity risk.

    - Monitor renal function when foscarnet is used with ritonavir, as this combination can lead to abnormal kidney function.

    - Avoid co-administration of QT-prolonging antiarrhythmic drugs (Class IA and III), such as quinidine, procainamide, dofetilide, amiodarone, and sotalol, as this raises the risk of QT prolongation and torsades de pointes.

Always discuss all current medications (including supplements and herbal products) with your healthcare provider before starting Foscavir.

Administration Considerations for Foscavir

Here’s how Foscavir is administered:¹

    - Foscarnet is usually given through a vein (IV infusion) because it doesn’t work well when taken by mouth.

    - The dose and speed of the drip depend on your age, weight, and the type of viral infection being treated.

    - In special cases (like certain eye infections), it may be injected directly into the eye by a trained doctor, but this is rare and only done by specialists.

    - Foscarnet must not be given as a quick injection. It should always be given slowly using a controlled infusion pump.

Accessing Foscavir in India

Foscavir is not commonly available in Indian pharmacies, but patients can still get it through a special system called the Named Patient Program. This allows individuals to access medicines that aren’t yet commercially available in the country. Through Rx4U, a reliable platform for such access, your doctor can request Foscavir specifically for your treatment. It’s important to have a valid prescription and medical need, as the process is regulated to ensure patient safety.

Frequently Asked Questions (FAQs) 

1. Who should not take Foscavir?

Foscavir may not be suitable for people with serious kidney problems, especially those on dialysis, as it can worsen kidney function. It should also be used with caution in older adults and children, since their bodies may react differently or retain the drug in bones and teeth. Pregnant or breastfeeding women should avoid it unless absolutely necessary, as its safety in these cases is unclear.1

2. Can pediatric patients receive Foscavir?

Foscarnet's safety in pediatric patients has not been established.1

3. Is there an oral form?

Foscavir is only available as an intravenous infusion. Oral bioavailability is not used in clinical practice.1

Note:

The information provided is for education purpose only and is subjected to prescribing information of the drug and the guidance of your treating physician. Always consult your health care provider before making any medical decision for starting your treatment.

Disclaimer:

Rx4U procures prescribed medicines directly from manufacturers or authorized distributors. It does not claim ownership of any trademarks and complies with the provisions of the Trademark Act, 1999, particularly Sections 30 and 30(1) concerning ‘Fair Use’. It solely facilitates access to new launches through named patient import.

References

1. Garikapati S, Nguyen M. [Updated 2024 Oct 29]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2025 Jan–. Available from: https://www.ncbi.nlm.nih.gov/books/NBK556108/