Tremfya (guselkumab) is an injection for adults with certain inflammatory conditions. It helps treat plaque psoriasis, psoriatic arthritis, ulcerative colitis, and Crohn's disease. Tremfya (guselkumab) is a monoclonal antibody known as an interleukin antagonist. It calms your immune system to lower inflammation. The dosing schedule is every 1 or 2 months, depending on your condition and how far along you are in treatment. Side effects are upper respiratory infections, headache, and injection site reactions. There's also a risk of other infections and liver problems as long-term side effects.