Allergic Reaction or Side Effect: How to tell the difference?

How do you tell the difference between itchy skin on 6mp caused by an allergic reaction, or just a side effect?

3 years ago I had no skin problems with it; this time around after re-starting it...I had itchyness which stopped when I stopped taking 6mp last week waiting for the lab results. They came back normal and the nurse said to re-start it...

I would call your doctor and let them know. With some meds, you develop adjectives reactions if you stop and restart them.

Oh I did the first week/last week They were worried about pancreatitis more but also any allergic reaction (abdominal pain and skin itchiness) so they told me to get labs and stop it until they get the results. This morning all came back normal so they said to restart it.