Is a side effect to januvia, (sitagliptin) one that messes with your allergies or sinus, if so does it last forever or not?

Rhino sinusitis: I assume by saying allergies, you mean nasal congestion, runny / itchy nose and sneezing. I think your assumption of januvia (sitagliptin) causing allergies or sinus symptoms is not correct. Medication allergies present differently like skin rashes, throat closing... And it goes away after you stop taking the medication. You may need to see a doctor for diagnosis & treatment of probably allergic rhino sinusitis.