Januvia (sitagliptin) impairs the immune system.

  Januvia helps treat diabetes by inhibiting the breakdown of incretins...hormones that increase insulin release in response to meals. It does this by blocking dipeptidyl peptidase-4 (DPP-4), an enzyme that inactivates incretins.

  Some experts are concerned there may be rare effects associated with other substrates of DPP-4. These may involve immune function including inhibition of T-cell activity.

  These concerns are fueled by reports of necrotic skin lesions in monkeys taking another "gliptin" up for approval called Galvus (vildagliptin).