Protease inhibitors act against the enzyme that HIV uses to break up large proteins into the smaller proteins from which the new viral particles are produced. New HIV particles produced in the presence of protease inhibitors are immature and non-infectious.

The most important area of HIV's protease enzyme is shaped like a pocket, and drugs must fit inside this pocket to block the activity of the enzyme. Small changes are constantly taking place in the structure of the enzyme when new copies of the virus are made, and sometimes these changes make it impossible for the protease inhibitor to block the action of the enzyme. This is called resistance. See Resistance to protease inhibitors in Anti-HIV therapy: Resistance for more information.

Nine protease inhibitors are now licensed treatments in Europe and the United States: amprenavir (Agenerase), atazanavir (Reyataz), fosamprenavir (Telzir), indinavir (Crixivan), lopinavir, nelfinavir (Viracept), ritonavir (Norvir), saquinavir (Invirase / Fortovase) and tipranavir (Aptivus).

To compare all antiretroviral drugs licensed in the European Union, see NAM's drug chart. The chart contains illustrations of the drugs, as well as information on drug doses, formulations, pill burdens, main side-effects and food restrictions.

Experimental protease inhibitors include TMC114.

For further information on individual protease inhibitors, see Drugs used by people with HIV: Protease inhibitors.