Foscarnet (phosphonomethanoic acid), known by its brand name Foscavir, is an antiviral medication which is primarily used to treat viral infections involving the Herpesviridae family.

[8] This phosphonic acid derivative (marketed by Clinigen as foscarnet sodium under the trade name Foscavir) is an antiviral medication used to treat herpes viruses, including drug-resistant cytomegalovirus (CMV) and herpes simplex viruses types 1 and 2 (HSV-1 and HSV-2).

Foscarnet can be used to treat highly treatment-experienced patients with HIV as part of salvage therapy.

[12][13] However, unlike acyclovir and ganciclovir, foscarnet is not activated by viral protein kinases, making it useful in acyclovir- or ganciclovir-resistant HSV and CMV infections.

[5] However, acyclovir- or ganciclovir-resistant mutants with alterations in viral DNA polymerase may also be resistant to foscarnet.