I'm Currently Taking Methimazole For My Over A Time Thyroid

I'm currently taking methimazole for my over a time thyroid and I found out that I have adult Add. My doctor wrote prescribed me Adderall. My pharmacist was upset and would not fill my medicine. He said there was a terrible side effect to mixing these two drugs together. What are the risk factors because neither the pharmacist nor my doctor will tell me. Tysm