Fosamprenavir (Telzir) is an anti-HIV drug that reduces the amount of virus in the body. Anti-HIV drugs such as fosamprenavir slow down or prevent damage to the immune system and reduce the risk of developing AIDS-related illnesses.

Fosamprenavir is a protease inhibitor. Protease (or proteinase) is the protein or enzyme that HIV uses to break up large viral proteins so new HIV particles can be made. For more information about how protease inhibitors work, see Protease inhibitors.

Fosamprenavir is a pro-drug of amprenavir (Agenerase), which means that it is converted into amprenavir in the body. Its chief advantage over amprenavir is that it requires fewer capsules each day, and achieves higher blood levels of the active drug. Amprenavir production was stopped in December 2004, so patients taking amprenavir are being encouraged to switch to fosamprenavir.

Fosamprenavir was previously known by the codename GW-433908. It was approved in the European Union in July 2004 under the brand name Telzir for use in combination with low-dose ritonavir (Norvir) in HIV-positive adults.

In the United States, fosamprenavir was approved for use with or without ritonavir boosting in October 2003, and is marketed under the trade name Lexiva. Since March 2004, United States guidelines have included fosamprenavir, with or without ritonavir boosting, as an option for treatment-naive patients.