List of products in the National Drug Code with proprietary name . This medication is used to treat plaque psoriasis and psoriatic arthritis. Guselkumab belongs to a class of drugs known as monoclonal antibodies. It works by blocking a certain natural substance in your body (interleukin-23) that may cause inflammation and swelling. Guselkumab can help lessen the red, scaly, itchy patches of skin caused by plaque psoriasis and lessen the pain and swelling in the joints caused by psoriatic arthritis.