“I’m a smoker. Would that be a reason I won’t pass medical exam?”
Just to clarify, if you’re the U.S. citizen, you will not have a medical exam. However, if you’re the alien, you will have a fiance visa medical exam. What they’re primarily looking for is communicable diseases and evidence of drug addiction or abuse. If you’re a tobacco smoker, even with lung problems related to it, that would not constitute something that will give you a problem. Lung problems like emphysema and lung cancer are not communicable/contagious.
Also, even though you are examined for evidence of illegal drug abuse, a drug test is not administered in most cases (as of August 2018 – this may change in the future). As of now, they would be looking at physical evidence such as needle marks, missing teeth, and other physical aspects like that.