Foscarnet sodium (Foscavir) is an antiviral drug that is active against viruses such as cytomegalovirus, Epstein-Barr virus, herpes viruses, varicella zoster virus and HIV. It works by inhibiting the DNA polymerases and reverse transcriptase. It is sometimes referred to as trisodium phosphonoformate or PFA.

Comparative studies have shown that foscarnet is as effective as ganciclovir (Cymevene) in treating cytomegalovirus retinitis, and is active against ganciclovir-resistant strains of cytomegalovirus.

Foscarnet is also as a treatment for aciclovir (Zovirax)-resistant herpes simplex and varicella zoster infections. It is also active against HIV^[1][2][3]. However, it is impractical for long-term use as an anti-HIV agent because it has to be given by intravenous infusion and causes substantial side-effects. Nevertheless, it may be an option for salvage therapy in patients with no other treatment options^[4]. Foscarnet can also be used to treat or prevent Kaposi’s sarcoma^[5].

The commonest side-effects of foscarnet are kidney toxicity, changes in the levels of minerals in the blood and anaemia. Other side-effects include neutropenia (low levels of neutrophils, a type of white blood cell), central nervous system toxicity such as neuropathy, muscle twitching and irregular heartbeat, diabetes, ulcers on the tip of the penis (especially in uncircumcised men), nausea, fever and rash. Treatment may cause a decline in the levels of calcium in the blood, which may in turn cause neurological and heart problems.

Patients receiving foscarnet who experience burning or tingling sensations or mouth numbness should tell a doctor immediately.

Patients need to drink plenty of water before or during infusion and for a saline infusion to be given at the same time as the drug, to reduce the risk of kidney damage. In men, careful washing of the penis after urination helps to minimise the risk of penile ulcers.

Foscarnet should not be taken at the same time as intravenous pentamidine (Pentacarinat) because of an increased risk of kidney damage. Caution is required when taking foscarnet with other drugs that may harm the kidneys, such as tenofovir (Viread) and amphotericin B (Fungilin / Fungizone / Abelcet / AmBisome / Amphocil). GlaxoSmithKline advises that 3TC (lamivudine, Epivir) should not be taken at the same time as foscarnet. It does not interact with any currently available protease inhibitors or non-nucleoside reverse transcriptase inhibitors (NNRTIs).