We plan to enrol 120 research participants in total in the two sites. The primary objective of the research project is to determine the change in the psoriasis area and severity index (ΔPASI) during twenty four weeks of treatment with a dipeptidyl peptidase-4 inhibitor (Januvia®, 100mg daily or 50mg daily for participants with moderate kidney disease) in psoriasis patients undergoing narrow-band ultraviolet-B (NB-UVB) light therapy. This will be compared to the ΔPASI of psoriasis patients undergoing NB-UVB light therapy who are allocated randomly to not receive any additional treatment.