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Medical Disclaimer: This information is for educational purposes only and is not a substitute for professional medical advice.
Other
Foscarnet is a potent antiviral medication used primarily to treat cytomegalovirus (CMV) retinitis in patients with AIDS and acyclovir-resistant herpes simplex virus infections. It belongs to the class of pyrophosphate analogues.
Name
Foscarnet
Raw Name
FOSCARNET SODIUM
Category
Other
Salt Form
Sodium
Drug Count
3
Variant Count
9
Last Verified
February 17, 2026
RxCUI
1734377, 855613
UNII
964YS0OOG1
About Foscarnet
Foscarnet is a potent antiviral medication used primarily to treat cytomegalovirus (CMV) retinitis in patients with AIDS and acyclovir-resistant herpes simplex virus infections. It belongs to the class of pyrophosphate analogues.
Detailed information about Foscarnet
This page is for informational purposes only and does not replace medical advice. Consult a qualified healthcare professional before using any medication containing Foscarnet.
Foscarnet was first approved by the U.S. Food and Drug Administration (FDA) in 1991. Since its introduction, it has remained a cornerstone of treatment for cytomegalovirus (CMV) retinitis (a viral infection of the retina that can lead to blindness) in patients with acquired immunodeficiency syndrome (AIDS). It is also frequently utilized for the treatment of mucocutaneous (affecting skin and mucous membranes) acyclovir-resistant herpes simplex virus (HSV) infections. Because of its potential for significant toxicity, particularly to the kidneys, healthcare providers typically reserve foscarnet for cases where other treatments have failed or are not tolerated.
At the molecular level, foscarnet exerts its antiviral activity by acting as a selective inhibitor of viral-specific DNA polymerases and reverse transcriptases. To understand how foscarnet works, it is helpful to visualize the process of viral replication. When a virus like CMV or HSV infects a human cell, it must copy its genetic material to produce new viral particles. This process requires an enzyme called DNA polymerase.
Foscarnet works by chemically mimicking pyrophosphate, a byproduct of the DNA synthesis process. It binds directly to the pyrophosphate-binding site on the viral DNA polymerase enzyme. By occupying this site, foscarnet prevents the enzyme from cleaving the pyrophosphate from the deoxynucleoside triphosphates (the building blocks of DNA). This effectively halts the elongation of the viral DNA chain. Because foscarnet binds directly to the enzyme and does not require phosphorylation (a chemical process of adding a phosphate group), it remains effective even if the virus has mutated to avoid other drugs that do require this step.
Importantly, foscarnet is highly selective. It inhibits viral DNA polymerase at concentrations that are much lower than those required to inhibit human cellular DNA polymerase. This selectivity is what allows the drug to kill the virus while ideally sparing the host's healthy cells, although some collateral effects on human cells contribute to its side effect profile.
Foscarnet is not well absorbed by the gastrointestinal tract. Consequently, it must be administered via intravenous (IV) infusion to achieve therapeutic levels in the bloodstream. When administered intravenously, the drug achieves immediate 100% bioavailability. The peak plasma concentrations (Cmax) are reached at the end of the infusion period.
Foscarnet exhibits a complex distribution pattern. It does not bind significantly to plasma proteins (less than 15-20% protein binding). One of the most notable features of foscarnet's distribution is its tendency to accumulate in the bone. Studies have shown that a significant portion of the administered dose is deposited in the skeletal system, where it may remain for extended periods. The drug has variable penetration into the cerebrospinal fluid (CSF), with levels ranging from 10% to 70% of plasma concentrations, depending on the integrity of the blood-brain barrier.
Foscarnet does not undergo significant metabolism by the liver. It is not a substrate for, nor does it inhibit or induce, the common cytochrome P450 (CYP) enzyme systems. This lack of metabolic processing reduces the risk of certain types of drug-drug interactions but places the entire burden of clearance on the kidneys.
The primary route of elimination for foscarnet is renal (kidney) excretion. Approximately 80% to 90% of a dose is excreted unchanged in the urine through glomerular filtration and tubular secretion. The plasma half-life of foscarnet is biphasic; the initial half-life is approximately 3 to 4 hours, but because the drug is released slowly from the bone, the terminal half-life can extend to several days or even weeks.
Foscarnet is FDA-approved for the following indications:
Off-label uses (uses not specifically approved by the FDA but supported by clinical practice) may include:
Foscarnet is available exclusively as an aqueous solution for intravenous infusion. It is typically supplied in a concentration of 24 mg/mL in 250 mL or 500 mL bottles. Because the solution is highly concentrated and can be irritating to the veins, it is often administered through a central venous catheter (a large vein in the chest) or diluted if administered through a peripheral vein.
> Important: Only your healthcare provider can determine if Foscarnet is right for your specific condition. The choice of antiviral therapy depends on the specific viral strain, your immune status, and your underlying kidney function.
Dosage for foscarnet is highly individualized and must be adjusted based on the patient's weight and renal (kidney) function. The treatment is typically divided into two phases: induction and maintenance.
Foscarnet is not formally FDA-approved for use in pediatric patients. However, it is sometimes used off-label in children who have life-threatening viral infections that are resistant to other therapies. When used in children, the dosage is generally calculated based on body weight (mg/kg) similar to adult dosing, but extreme caution is required due to the potential effects of foscarnet on developing bones and teeth. Pediatric specialists must carefully weigh the risks and benefits in this population.
Renal function must be assessed before starting foscarnet and monitored frequently (often 2-3 times per week) during therapy. Dose adjustments are mandatory if the creatinine clearance (a measure of kidney function) drops. If renal function worsens significantly, the drug may need to be discontinued temporarily or the dose reduced according to a specific sliding scale provided in the manufacturer's labeling.
Since foscarnet is not metabolized by the liver, no specific dosage adjustments are required for patients with liver disease. However, these patients should still be monitored for overall stability.
Elderly patients are more likely to have decreased renal function. Therefore, the dose should be selected cautiously, starting at the lower end of the dosing range, and renal function must be monitored with high frequency.
Foscarnet is administered only by intravenous infusion, typically in a hospital or specialized infusion center. It should never be given by rapid IV bolus injection, as this significantly increases the risk of toxicity.
Because foscarnet is administered by healthcare professionals, missed doses are rare. However, if an infusion is delayed or missed, it should be administered as soon as possible. Do not double the dose to catch up. Maintaining a consistent schedule is vital for suppressing viral replication.
Signs of foscarnet overdose may include acute kidney failure, seizures, and severe electrolyte imbalances (such as dangerously low calcium or magnesium levels). In the event of an overdose, the infusion must be stopped immediately. Treatment is supportive, focusing on vigorous hydration and the correction of electrolyte levels. Hemodialysis may be effective in removing foscarnet from the blood in cases of severe overdose or renal failure.
> Important: Follow your healthcare provider's dosing instructions. Do not adjust your dose or skip infusions without medical guidance, as this can lead to viral resistance.
Foscarnet is associated with a high incidence of side effects, many of which are related to its chemical properties and its effect on mineral metabolism.
> Warning: Stop taking Foscarnet and call your doctor immediately if you experience any of these.
Because foscarnet accumulates in the bone, there are theoretical concerns regarding long-term bone health. In children, this could potentially affect bone growth. In adults, the long-term consequences are less clear, but chronic use is generally avoided due to the cumulative risk of nephrotoxicity (kidney damage). Prolonged use may also lead to the development of viral resistance, making the drug less effective over time.
The FDA has issued a Black Box Warning for foscarnet regarding two major risks:
Report any unusual symptoms to your healthcare provider immediately. Regular blood tests are a mandatory part of foscarnet therapy.
Foscarnet is a high-alert medication that requires intensive clinical management. It is not a cure for CMV or HSV; rather, it suppresses the virus to prevent disease progression. Patients must remain under the close supervision of a physician experienced in treating opportunistic infections.
REVISED FDA BOXED WARNING (Summary):
This is the most significant risk associated with foscarnet. The drug can cause crystal nephropathy (formation of crystals in the kidney tubules) or direct toxic effects on the renal cells. Patients with pre-existing kidney disease are at much higher risk. Healthcare providers will check your kidney function at least two to three times per week during induction therapy.
Foscarnet is a chelating agent, meaning it binds to minerals in the blood. This can cause sudden and severe drops in ionized calcium, which can lead to tetany (muscle spasms) or cardiac arrest. Magnesium, potassium, and phosphorus levels are also frequently affected. Supplements are often prescribed alongside foscarnet to maintain these levels.
Because foscarnet is deposited in the bone, it may interfere with the normal turnover of bone minerals. This is of particular concern in pediatric patients or those with metabolic bone diseases.
Foscarnet can cause anemia (low red blood cells) and leukopenia (low white blood cells). While less common than with ganciclovir, regular complete blood counts (CBC) are necessary.
While receiving foscarnet, you will require frequent laboratory testing, including:
Foscarnet can cause dizziness, seizures, and visual changes. You should not drive or operate heavy machinery until you know how the medication affects you. If you experience any neurological symptoms, avoid these activities entirely.
There are no direct chemical interactions between alcohol and foscarnet. However, alcohol can cause dehydration and electrolyte shifts, which may worsen foscarnet's toxicity to the kidneys and brain. It is generally recommended to avoid alcohol during treatment.
Stopping foscarnet suddenly can lead to a rapid rebound of the viral infection, which may cause permanent vision loss (in CMV retinitis) or severe skin lesions (in HSV). Do not stop the medication unless directed by your doctor. If the drug must be stopped due to toxicity, your doctor will transition you to an alternative therapy if possible.
> Important: Discuss all your medical conditions, especially kidney disease or a history of seizures, with your healthcare provider before starting Foscarnet.
Foscarnet should never be used concurrently with other highly nephrotoxic (toxic to the kidneys) medications unless absolutely necessary and under extreme supervision.
Using foscarnet with other drugs that damage the kidneys increases the risk of permanent renal failure. Examples include:
Since foscarnet is administered intravenously, there are no direct interactions with food absorption. However, maintaining a diet rich in minerals (calcium, magnesium, potassium) may be recommended by your dietitian to help offset the losses caused by the medication. Ensure you are drinking plenty of non-alcoholic fluids to stay hydrated.
For each major interaction, the mechanism is typically either synergistic toxicity (two drugs damaging the same organ) or pharmacodynamic interference (two drugs affecting the same mineral levels). The management strategy always involves increased frequency of laboratory monitoring or the selection of an alternative, less toxic medication.
> Important: Tell your doctor about ALL medications, supplements, and herbal products you are taking, including over-the-counter pain relievers.
Foscarnet must NEVER be used in the following circumstances:
These conditions require a careful risk-benefit analysis by the healthcare team:
There is no known cross-sensitivity between foscarnet and other common antiviral drugs like acyclovir, ganciclovir, or cidofovir. This is because foscarnet is chemically unrelated to these nucleoside analogues. Therefore, foscarnet is often the drug of choice for patients who have failed or are allergic to those other medications.
> Important: Your healthcare provider will evaluate your complete medical history, including your kidney function and neurological health, before prescribing Foscarnet.
Foscarnet is classified as FDA Pregnancy Category C. This means that animal studies have shown adverse effects on the fetus (such as skeletal abnormalities), but there are no adequate, well-controlled studies in humans. Foscarnet is known to cross the placenta and accumulate in the fetal skeleton. It should be used during pregnancy only if the potential benefit justifies the potential risk to the fetus. If a pregnant woman requires foscarnet for CMV retinitis, close monitoring of the fetus via ultrasound is recommended.
It is not known whether foscarnet is excreted in human breast milk. However, due to the potential for serious adverse reactions in nursing infants (including kidney damage and bone effects), and because the virus (HIV) for which foscarnet is often prescribed can be transmitted through breast milk, breastfeeding is generally not recommended for women receiving foscarnet.
The safety and effectiveness of foscarnet in children have not been established. The primary concern in the pediatric population is the deposition of foscarnet in developing bone and teeth. Animal studies have shown that foscarnet can affect tooth enamel and bone density. If foscarnet must be used in a child, pediatric infectious disease specialists and nephrologists must be involved in the care to monitor for growth disturbances and renal toxicity.
Clinical studies of foscarnet did not include sufficient numbers of subjects aged 65 and over to determine whether they respond differently than younger subjects. However, it is well-known that renal function declines with age. Since foscarnet is cleared entirely by the kidneys, the risk of toxic reactions is significantly higher in elderly patients. Dose selection should be conservative, and renal function must be checked frequently.
Renal impairment is the most common reason for foscarnet dose modification or discontinuation.
Because the liver is not involved in the metabolism or excretion of foscarnet, hepatic impairment does not significantly alter the pharmacokinetics of the drug. No specific dose adjustments are provided for patients with liver disease, though they should be monitored for general systemic stability.
> Important: Special populations require individualized medical assessment and often more frequent laboratory monitoring than the general population.
Foscarnet is an inorganic pyrophosphate analogue. Its primary mechanism of action is the non-competitive inhibition of viral-specific DNA polymerase and reverse transcriptase.
Unlike nucleoside analogues (like acyclovir), foscarnet does not require activation by viral thymidine kinase or other cellular kinases. It binds directly to the pyrophosphate-binding site of the viral DNA polymerase. This binding prevents the cleavage of pyrophosphate from deoxynucleoside triphosphates, which is a necessary step for the elongation of the DNA chain. By inhibiting this step, foscarnet prevents the virus from replicating its genome. It is active against all known herpes viruses (CMV, HSV-1, HSV-2, VZV, EBV, and HHV-6) and some retroviruses, including HIV.
The antiviral effect of foscarnet is concentration-dependent. The relationship between the plasma concentration and the inhibition of viral replication is well-defined for CMV and HSV. Because it is a non-competitive inhibitor, it can remain effective even when the natural building blocks of DNA are present in high concentrations. Tolerance to foscarnet does not typically develop in the traditional sense, but viral resistance can occur through mutations in the viral DNA polymerase gene.
| Parameter | Value |
|---|---|
| Bioavailability | 100% (Intravenous) |
| Protein Binding | 14% - 17% |
| Half-life (Initial) | 3 - 4 hours |
| Half-life (Terminal) | 18 - 88 hours (due to bone release) |
| Tmax | End of infusion |
| Metabolism | Not metabolized |
| Excretion | Renal 80% - 90% (unchanged) |
Foscarnet is classified as a Pyrophosphate Analogue Antiviral. It is currently the only member of this class in widespread clinical use. It is distinct from nucleoside analogues (like Ganciclovir) and nucleotide analogues (like Cidofovir).
Medications containing this ingredient
Common questions about Foscarnet
Foscarnet is a potent antiviral medication primarily used to treat cytomegalovirus (CMV) retinitis in patients with AIDS. It is also a critical treatment for herpes simplex virus (HSV) infections that have become resistant to acyclovir in immunocompromised individuals. Because it works differently than other antivirals, it is often used as a 'second-line' or 'rescue' therapy when other drugs fail. It is administered only by intravenous infusion, usually in a hospital or clinic setting. Your doctor will determine if it is necessary based on the severity of your infection and your previous response to other treatments.
The most common side effects of foscarnet include kidney dysfunction, nausea, fever, and significant changes in blood minerals such as calcium, magnesium, and potassium. Many patients also experience headaches and anemia (low red blood cell counts). Because foscarnet is excreted in the urine, it can also cause painful sores or ulcers in the genital area if the area is not kept clean after urinating. These side effects are often dose-dependent and require frequent blood tests to monitor. Most side effects improve once the medication is stopped or the dose is adjusted.
It is generally advised to avoid alcohol while receiving foscarnet treatment. While there is no direct chemical interaction between the two, alcohol can cause dehydration and alter your body's electrolyte balance. Since foscarnet is highly toxic to the kidneys and can cause seizures related to electrolyte shifts, alcohol consumption can significantly increase these risks. Staying well-hydrated with water and electrolyte-balanced fluids is essential for protecting your kidneys during therapy. Always discuss your lifestyle and habits with your healthcare provider before starting treatment.
Foscarnet is not considered safe for use during pregnancy unless the potential benefits clearly outweigh the risks to the fetus. It is classified as FDA Category C, meaning animal studies have shown potential harm to the developing skeleton, but human data is lacking. The drug is known to cross the placenta and accumulate in fetal bones. If you are pregnant or planning to become pregnant, you must inform your doctor immediately. They will likely explore safer antiviral alternatives unless foscarnet is the only option to save your vision or treat a life-threatening infection.
In the treatment of CMV retinitis, foscarnet usually begins to stabilize retinal lesions within 2 to 3 weeks of starting the induction phase. For herpes simplex infections, improvement in skin lesions is typically seen within 10 to 14 days. However, the 'induction' phase of treatment usually lasts at least 21 days to ensure the virus is sufficiently suppressed. Even after the infection appears to be under control, many patients must continue a lower 'maintenance' dose indefinitely to prevent the virus from returning. Your progress will be monitored through regular eye exams or physical assessments.
You should never stop taking foscarnet suddenly without consulting your healthcare provider. Foscarnet does not cure viral infections; it only suppresses them. If the medication is stopped prematurely, the virus can rapidly begin replicating again, which may lead to a worsening of symptoms, permanent vision loss, or the development of drug-resistant viral strains. If side effects become too severe, your doctor will carefully manage the transition to a different medication. Always follow the full course of treatment as prescribed by your medical team.
Since foscarnet is administered by healthcare professionals in a clinical setting, missing a dose is unlikely. However, if you miss an appointment for your infusion, contact your healthcare provider or infusion center immediately to reschedule. Consistency is vital for keeping the virus suppressed. Do not attempt to 'double up' on doses or change your schedule without medical approval. Your doctor will advise you on the best way to get back on track with your treatment plan to ensure the infection does not flare up.
Weight gain is not a typical side effect of foscarnet. In fact, many patients may experience weight loss due to side effects like nausea, vomiting, or a general loss of appetite during the treatment period. If you notice sudden weight gain or swelling (edema) in your legs, ankles, or face, you should contact your doctor immediately. This could be a sign of kidney impairment, as the body may be retaining fluid because the kidneys are not functioning properly. Your healthcare provider will monitor your weight and fluid balance throughout the course of your therapy.
Foscarnet has many serious interactions with other medications, particularly those that also affect the kidneys. You must inform your doctor of every medication you are taking, including over-the-counter drugs like ibuprofen (Advil, Motrin) and supplements. Taking foscarnet with certain antibiotics (like gentamicin) or antifungals (like amphotericin B) can lead to severe kidney damage. The combination of foscarnet and intravenous pentamidine is particularly dangerous and must be avoided. Your pharmacist and doctor will perform a thorough drug interaction check before you begin your first infusion.
Yes, foscarnet sodium is available as a generic medication. The brand name version, Foscavir, was the original formulation, but several manufacturers now produce generic versions of the intravenous solution. Generic foscarnet is required to meet the same FDA standards for safety, purity, and effectiveness as the brand-name drug. Because it is an injectable medication used for complex infections, it is typically stocked by hospital pharmacies or specialized home infusion companies rather than standard retail pharmacies. Your insurance provider can help determine the coverage and cost of the generic version.