After a three week run-in period, where research participants will not receive the investigational medicinal product, research participants will be allocated randomly, after stratification by centre, by HbA1c level (HbA1c < 38mmol/mol or ≥ 38mmol/mol), BMI level (BMI < 30 kg/m2 or ≥ 30 kg/m2) and previous response to NB-UVB (achieved remission within 25 exposures during most recent course of NBUVB or not) either to Arm A or to Arm B. Those who have not been exposed previously to NBUVB will be placed in the "did not achieve remission within 25 exposures" group. Research participants allocated to Arm A will receive a twenty six week supply of Januvia® tablets (DPP-4 Inhibitor) and will be instructed to ingest orally one 100mg tablet once daily (or 50mg once daily for participants with moderate kidney disease) for twenty four weeks. Research participants allocated to Arm B will receive no treatment (aside from usual medications and phototherapy). Both the research participants and the investigators will be aware of the trial arm to which the research participant has been allocated randomly (open-label study).