Protease inhibitors are a class of medication used in viral infections. They act by inhibiting specific viral enzymes that are essential to the viral life cycle by cleaving viral proteins.

There are various viruses for which protease inhibitors have been developed (or are presently undergoing testing):

• HIV/AIDS: antiretroviral protease inhibitors (saquinavir, ritonavir, indinavir, nelfinavir etc.)
• Hepatitis C: experimental agents: BILN 2061, VX 950.

Given the specificity of the target of these drugs there is the risk, as in antibiotics, of the development of drug-resistant mutated viruses. To reduce this risk it is common to use together different drugs aimed at different targets.