Fosamprenavir is an antiviral drug that treats patients with HIV. Fosamprenavir is always combined with other anti-HIV medications. It blocks the multiplication of HIV cells that results to AIDS. This medicine cures neither HIV nor AIDS. Fosamprenavir belongs to a group of anti-HIV drugs called protease inhibitors and has a brand name Lexiva. It works by reducing the quantity of HIV in the blood, enhancing your immune system.