Protease inhibitor: An agent that can keep a protease from splitting a protein into peptides. Examples of protease inhibitors are saquinavir (brand name: Invirase) and ritonavir (brand name: Norvir), and they are used primarily in HIV/AIDS treatment. They are taken as part of a multi-drug cocktail and have been shown to be capable of significantly reducing the level of HIV virus in the blood. Side effects associated with protease inhibitors include lipodystrophy syndrome, in which the face, arms, and legs become thin due to loss of subcutaneous fat; the skin becomes dry; weight loss occurs; and abnormal deposits of fat occur. Some strains of HIV may be resistant to protease inhibitors.