Stelara (ustekinumab) is a monoclonal antibody used to treat adult patients with moderate-to-severe plaque psoriasis, active psoriatic arthritis, and moderate-to-severe Crohn’s disease. The medication works by blocking the activity of cytokines; interleukin-12 (IL-12) and interleukin-23 (IL-23), which are involved in the immune system activation and inflammation.