Hi Becky,   From what I could find, guanfacine has no known interactions with Singulair, Flonase or Qvar. And those allergy and asthma medicines are all frequently used with each other and are safe together. So, you should be able to stop worrying on that account.   However, if you do have any concerns about unexpected effects in the future, be sure to discuss them with each of your son's doctors. Also, make sure each doctor know what the others have prescribed. Your local pharmacist is also a great resource about drug interactions and side effects and should be happy to answer questions.   Best, Kathi