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Pyrophosphate antiviral Pregnancy: Not recommended during pregnancy; should not be used during breast-feeding. UK SPC section 4.6: 'There are no or limited amount of data from the use of foscarnet in pregnant women. Animal studies are insufficient with respect to reproductive toxicity. Foscavir is not recommended during pregnancy.' Lactation: 'There is insufficient information on the excretion of foscarnet in human milk. Available pharmacodynamic/toxicological data in animals have shown excretion of foscarnet in milk. A risk to the newborns/infants cannot be excluded. Foscavir should not be used during breast-feeding.' Contraception: 'Women capable of childbearing should use effective contraception methods during Foscavir therapy. Men treated with Foscavir should not father a child during or up to 6 months after therapy.' Fertility: no data available in humans; no effects on fertility were observed in animal studies.

Foscarnet sodium

Brand names: Foscavir

Foscarnet sodium is an intravenous antiviral of the pyrophosphate analogue class, used for cytomegalovirus disease (notably CMV retinitis in immunocompromised patients) and aciclovir-resistant herpesvirus infections.

Auto-extracted from the source labelling — not yet independently clinician-verified. These values were distilled from the UK SPC (or the US label where noted) but have not had a clinician sign-off. Confirm against the current SmPC before prescribing.

Adult dose

Dose: CMV RETINITIS, INDUCTION (normal renal function): 60 mg/kg every 8 hours, given over 2 to 3 weeks depending on the clinical response. CMV RETINITIS, MAINTENANCE (following induction): seven days a week for as long as therapy is considered appropriate; initiate at 60 mg/kg once daily, and an increase to 90 to 120 mg/kg may then be considered in patients tolerating the initial dose level and/or those with progressive retinitis (a number of patients have received 90 mg/kg over a 2 hour period as a starting dose for maintenance therapy). Patients who experience progression of retinitis while on maintenance may be re-treated with the induction regimen. MUCOCUTANEOUS HSV INFECTION UNRESPONSIVE TO ACICLOVIR, INDUCTION: 40 mg/kg over one hour every 8 hours for 2 to 3 weeks or until healing of lesions - efficacy of maintenance therapy after HSV induction has not been established. Dosage MUST be individualised for the patient's renal function using the creatinine clearance chart (see renalAdjustment).
Route: Intravenous infusion only, by a central venous line or a peripheral vein. When peripheral veins are used, the 24 mg/ml solution must be diluted with equal parts of 0.9% sodium chloride (9 mg/ml) or 5% dextrose (50 mg/ml) by the hospital pharmacy; the 24 mg/ml solution may be given without dilution via a central vein. The infusion time should not be shorter than 1 hour. Do not administer by rapid intravenous injection.
Frequency: Induction (both CMV and HSV): every 8 hours. CMV maintenance: one infusion daily, seven days a week.
Max: 180 mg/kg/day - the maximum recommended daily dose of this product, stated in the SPC as 12 g of Foscavir per day, equivalent to 180 mg/kg/day in an average 70 kg male
UK SPC (Foscavir 24 mg/ml Solution for Infusion) section 4.2, captured complete. CMV INDUCTION, verbatim: 'Foscavir is administered over 2-3 weeks depending on the clinical response, as intermittent infusions every 8 hours at a dose of 60 mg/kg in patients with normal renal function. Dosage must be individualised for patient's renal function (see dosing chart below). The infusion time should not be shorter than 1 hour.' MAINTENANCE, verbatim: 'For maintenance therapy, following induction therapy of CMV retinitis, Foscavir is administered seven days a week as long as therapy is considered appropriate. In patients with normal renal function, it is recommended to initiate therapy at 60 mg/kg. Increase to a dose of 90-120 mg/kg may then be considered in patients tolerating the initial dose level and/or those with progressive retinitis. A number of patients have received 90 mg/kg over a 2 hour period as a starting dose for maintenance therapy.' HSV, verbatim: 'Foscavir is administered for 2-3 weeks or until healing of lesions, as intermittent infusions at a dose of 40 mg/kg over one hour every 8 hours in patients with normal renal function.' || HYDRATION - renal toxicity can be reduced by adequate hydration: 'It is recommended to establish diuresis by hydration with 0.5-1.0 litre of normal saline at each infusion. In compliant patients, oral hydration with similar hydration regimens has been used. Clinically dehydrated patients should have their condition corrected before initiating Foscavir therapy.' || HAEMODIALYSIS: 'Foscavir is not recommended in patients undergoing haemodialysis since dosage guidelines have not been established.' || ELDERLY: 'As for adults.' HEPATIC: 'Dose adjustment is not required in patients with hepatic insufficiency.' || SODIUM LOAD (section 4.4): the product contains 1.38 g of sodium per 250 ml bottle, equivalent to 69% of the WHO recommended maximum daily intake of 2 g sodium for an adult; the maximum recommended daily dose of 12 g per day is equivalent to 138% of that intake. Foscavir is considered high in sodium - take particular account in patients on a low sodium diet, and avoid its use when a saline load cannot be tolerated (e.g. in cardiomyopathy). || TRUNCATION CAVEAT: sections 4.2, 4.3 and 4.6 were captured complete; section 4.4 was cut at the source-fetch limit part-way through the genital irritation/ulceration paragraph and section 4.8's table was cut at 'Reproductive system and breast' disorders, so the warnings and side-effect lists below are NOT complete.

Paediatric dose

Route:
NO PAEDIATRIC DOSE EXISTS IN EITHER FETCHED LABEL, so dosePerKg is null and the weight calculator is deliberately unset. UK SPC section 4.2 verbatim: 'Paediatric population: The safety and efficacy of foscarnet in children have not been established. Please refer to sections 4.4 and 5.3.' Section 4.4 verbatim: 'Foscavir is deposited in teeth, bone and cartilage. Animal data show that deposition is greater in young animals. The safety of Foscavir and its effect on skeletal development have not been investigated in children.' The US label agrees: 'The safety and effectiveness of foscarnet sodium injection in pediatric patients have not been established. Foscarnet sodium injection is deposited in teeth and bone and deposition is greater in young and growing animals. Foscarnet sodium injection has been demonstrated to adversely affect development of tooth enamel in mice and rats... Administration to pediatric patients should be undertaken only after careful evaluation and only if the potential benefits for treatment outweigh the risks.' Do NOT scale the adult mg/kg regimen for a child. Verify against a children's formulary.

Dose adjustments

Renal

Dose MUST be individualised to creatinine clearance using the SPC dosing chart (Table 1), expressed in ml/kg/min. INDUCTION THERAPY, dose every 8 hours - CrCl above 1.6: CMV 60 mg/kg, HSV 40 mg/kg. 1.6 to 1.4: CMV 55, HSV 37. 1.4 to 1.2: CMV 49, HSV 33. 1.2 to 1.0: CMV 42, HSV 28. 1.0 to 0.8: CMV 35, HSV 24. 0.8 to 0.6: CMV 28, HSV 19. 0.6 to 0.4: CMV 21, HSV 14. Below 0.4: treatment not recommended. CMV MAINTENANCE THERAPY, one infusion dose in mg/kg/day given in not less than one hour - CrCl above 1.6: 60 (a number of patients have received 90 mg/kg as a starting dose for maintenance therapy). 1.6 to 1.4: 55. 1.4 to 1.2: 49. 1.2 to 1.0: 42. 1.0 to 0.8: 35. 0.8 to 0.6: 28. 0.6 to 0.4: 21. Below 0.4: treatment not recommended. Foscavir is not recommended in patients undergoing haemodialysis since dosage guidelines have not been established. Serum creatinine should be monitored every second day during induction therapy and once weekly during maintenance therapy, with appropriate dose adjustments performed according to renal function.

Hepatic

Dose adjustment is not required in patients with hepatic insufficiency (UK SPC section 4.2).

Dose auto-extracted from UK Summary of Product Characteristics (SPC) via the eMC; US FDA prescribing information (openFDA / DailyMed) — cross-check; US labelling may differ from UK — not yet clinician-verified. Always confirm against the product SmPC and your local formulary before prescribing.

Contraindications

  • Hypersensitivity to the active substance or to any of the excipients (UK SPC section 4.3)
  • Treatment is not recommended when creatinine clearance is below 0.4 ml/kg/min - the SPC dosing chart states 'Treatment not recommended' for that band, for both CMV and HSV induction and for CMV maintenance
  • Not recommended in patients undergoing haemodialysis, since dosage guidelines have not been established (section 4.2)
  • Not recommended during pregnancy; should not be used during breast-feeding (section 4.6)
  • Avoid use when a saline load cannot be tolerated, e.g. in cardiomyopathy - the product is high in sodium and requires saline hydration with each infusion (section 4.4)

Side effects

  • Very common: granulocytopenia, anaemia; decreased appetite, hypokalaemia, hypomagnesaemia, hypocalcaemia; dizziness, headache, paraesthesia; diarrhoea, nausea, vomiting; rash
  • Renal impairment is the major toxicity - common: renal impairment, acute renal failure, dysuria, polyuria, proteinuria; uncommon: glomerulonephritis, nephrotic syndrome; not known: renal pain, renal tubular acidosis, crystal nephropathy, haematuria. The US label quantifies this: approximately 33% of 189 patients with AIDS and CMV retinitis given 60 mg/kg three times daily WITHOUT adequate hydration developed serum creatinine of 2.0 mg/dL or above, falling to 12% (34/280) in subsequent trials where 1,000 mL of normal saline or 5% dextrose was given with each infusion
  • Electrolyte disturbance - foscarnet chelates bivalent metal ions such as calcium, so administration may be associated with an acute decrease of ionised serum calcium proportional to the rate of infusion, which may not be reflected in total serum calcium levels (section 4.4). Common: hyperphosphataemia, hyponatraemia, hypophosphataemia, hypercalcaemia, dehydration, raised alkaline phosphatase and LDH; uncommon: acidosis; not known: hypernatraemia
  • Seizures related to alterations in plasma minerals and electrolytes; cases of status epilepticus have been reported (section 4.4). Common: convulsion. The US label reports seizures in 18/189 (10%) of AIDS patients in the initial five controlled studies, with risk factors of impaired baseline renal function, low total serum calcium and underlying CNS conditions
  • Cardiac: common palpitations and tachycardia; not known - QT prolongation, ventricular arrhythmia and torsade de pointes
  • Common haematological: leukopenia, thrombocytopenia, neutropenia; uncommon pancytopenia
  • Common: sepsis; aggression, agitation, anxiety, confusional state, depression, nervousness; abnormal coordination, hypoaesthesia, involuntary muscle contractions, peripheral neuropathy, tremor; hypertension, hypotension, thrombophlebitis; abdominal pain, constipation, dyspepsia, pancreatitis, gastrointestinal haemorrhage; abnormal hepatic function; pruritus; myalgia
  • Not known: hypersensitivity including anaphylactic reactions and anaphylactoid reactions; diabetes insipidus; oesophageal ulceration; erythema multiforme, toxic epidermal necrolysis, Stevens-Johnson syndrome; muscular weakness, myopathy, myositis, rhabdomyolysis. Uncommon: urticaria, angioedema
  • Genital irritation and/or ulceration - foscarnet is excreted in high concentrations in the urine and may be associated with significant genital irritation and/or ulceration (section 4.4, truncated at the source-fetch limit)
  • Section 4.8 caveat: 'in these clinical trials, hydration and attention to electrolyte balance was not consistently given; the frequency of some adverse events will be lower when current recommendations are followed'
  • INCOMPLETE: the section 4.8 table was cut at the source-fetch limit at 'Reproductive system and breast' disorders - consult the full SPC

Interactions

  • Potentially nephrotoxic medicinal products - the renal function of patients receiving concomitant nephrotoxic drugs must be closely monitored (UK SPC section 4.4). The US label advises avoiding combination with aminoglycosides, amphotericin B, ciclosporin, aciclovir, methotrexate, tacrolimus and intravenous pentamidine unless the potential benefits outweigh the risks
  • Intravenous pentamidine - a possible interaction has been described; concomitant treatment of four patients in the United Kingdom may have caused hypocalcaemia, and one patient died with severe hypocalcaemia. Renal impairment and symptomatic hypocalcaemia have been observed during concurrent treatment (US labelling). Toxicity with aerosolised pentamidine has not been reported
  • Other drugs known to influence serum calcium levels - extreme caution, because foscarnet can reduce serum levels of ionised calcium (US labelling)
  • Medicines known to prolong the QT interval - foscarnet has been associated with QT prolongation and, more rarely, torsade de pointes; monitor carefully (UK SPC section 4.4)
  • Diuretics - when diuretics are indicated, thiazides are recommended over loop diuretics, because loop diuretics inhibit renal tubular secretion and may impair elimination of foscarnet, potentially leading to toxicity (US labelling)

Monitoring

  • Serum creatinine every second day during induction therapy and once weekly during maintenance therapy, with dose adjusted according to renal function - renal function impairment may occur at any time during administration
  • Hydration - maintain adequate hydration in all patients; establish diuresis with 0.5 to 1.0 litre of normal saline at each infusion, and correct clinical dehydration before starting therapy
  • Electrolytes, especially calcium and magnesium, assessed prior to and during therapy, with deficiencies corrected; mineral and electrolyte supplementation may be required
  • Monitor carefully for ventricular arrhythmia in patients with known prolongation of cardiac conduction intervals (particularly QTc), significant electrolyte disturbance (hypokalaemia, hypomagnesaemia), bradycardia, underlying cardiac disease such as congestive heart failure, or concomitant QT-prolonging medicines; advise patients to report cardiac symptoms promptly
  • Monitor for seizures and their potential sequelae, which relate to alterations in plasma minerals and electrolytes
  • Infusion control - never give by rapid or bolus intravenous injection; the US label states that an infusion pump must be used, and that overdoses have occurred in spite of pump use
  • Sodium load - account for 1.38 g of sodium per 250 ml bottle, particularly in patients on a low sodium diet

Clinical monograph

How it works

It reversibly blocks the pyrophosphate binding site of viral DNA polymerase and reverse transcriptase, inhibiting viral nucleic acid synthesis without requiring intracellular activation by viral kinases.

Prescribing in practice

  • Nephrotoxicity is the principal dose-limiting toxicity; maintain adequate hydration, monitor renal function closely and adjust the regimen for renal impairment.
  • Causes electrolyte disturbances including hypocalcaemia, hypomagnesaemia, hypokalaemia and hyperphosphataemia or hypophosphataemia, which can precipitate arrhythmias and seizures.
  • Administer by controlled infusion through a suitable line and consult the SPC and current prescribing references for reconstitution and infusion-rate requirements.

Monitoring

Monitor renal function and serum electrolytes (including calcium, magnesium, potassium and phosphate) regularly throughout treatment, with more frequent checks during induction.

Counselling the patient

  • Report numbness, tingling around the mouth, muscle twitching or seizures, as these may indicate electrolyte disturbance.
  • Maintaining good fluid intake helps protect the kidneys during treatment.
  • Attend all scheduled blood tests and ophthalmology reviews as advised.

Evidence & guidelines

Foscarnet is an established option for CMV disease and aciclovir-resistant herpesvirus infection in immunocompromised patients, as reflected in UK and international guidance.

Reference: SmPC; BHIVA; Drug verified in RxNorm (NLM); confirm dosing against the manufacturer SPC (eMC). Verify against your local formulary and current prescribing references before prescribing. The structured dose values shown have been reviewed by a clinician. Monograph status: clinician-reviewed (2026-07-04).

Related

Curated clinical cross-links plus same-class fallbacks.

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