Which of the following medications is commonly used to suppress the immune system?

Tacrolimus is an immunosuppressive medication that is primarily used to prevent organ rejection in transplant patients and to treat certain autoimmune conditions. It works by inhibiting T-lymphocyte activation and proliferation, which are critical processes in the body's immune response. The suppression of the immune system is essential in transplant patients to prevent their bodies from rejecting new organs, as a robust immune response can lead to an attack on the foreign tissue.

Other medications listed serve very different purposes. For instance, azithromycin is an antibiotic used to treat bacterial infections, metformin is primarily used to manage type 2 diabetes by improving insulin sensitivity, and lisinopril is an antihypertensive agent that helps manage high blood pressure by blocking angiotensin II's effects. These medications do not have the primary function of suppressing the immune system.