Losartan is an angiotensin II receptor antagonist, so it is a hypertensive agent. Angiotensin II is naturally present in the body but it can cause constriction of the blood vessels which increases blood pressure, hypertension. Losartan is therefore used to lower mild to moderate forms of hypertension.

Various studies have shown that the ACE2 protein could be the gateway for the SARS-CoV-2 virus to enter the human body and therefore act as a receptor for SARS-CoV-2 which is the cause of Covid-19. Angiotensin converting enzyme II (ACE2) is a protein (carboxypeptidase) homologous to the angiotensin converting enzyme (historically called ACE, which should now be renamed ACE1). ACE2 negatively regulates the renin-angiotensin system and acts as an angiotensin II clearance protein by converting it to angiotensin which is an active peptide but has properties opposite to those of angiotensin II. A study conducted in mice showed a decrease in the level of ACE2 in mouse lungs after injection of CoV-2-CoV-SARS as well as a deterioration of respiratory functions. Administration of losartan would have improved respiratory function in these same mice by restoring normal levels of ACE2.