I have been on sitagliptin now for over four years now, and no side effects.
But as with most drugs, it is so individual, who can cope and those that do have side effects. If it used to lower blood glucose levels, because the drug improves initial insulin response to offset the spike derived from food, then if you do have side effects, it can be changed to other diabetic Gliptin drugs or other blood glucose lowering meds.
I would like to add, if you believe something will happen, it probably will.
So, if you are anxious about the drug, then your experience of the drug will be counterproductive and counterintuitive. So if you start taking the drug, give it time to work for you, because if you wake up on the day after or third day with a sniffle, you will blame the drug, whereas, you may have got the cold before taking it. It may have nothing to do with the drug.
Only when the symptoms persist and a real change in your wellness and wellbeing happening should you go back to your GP.
Best wishes