Hi.. Can understand your concern.. Chantix contains drug varenicline and is used quit smoking when prescribed with proper counselling and behavioural modifications. Chest tightness,difficulty in breathing,forceful breathing are all side effects of Chantix. And if you are experiencing these side effects you should immediately stop taking the medicine and consult your doctor soon so that he can replace your medicine with other medicines like Zyban. But you should not take any medicine without consulting a doctor. Hope your query is solved.. Thanks and regards..