"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 foreign partner, you have to take a fiancé visa medical exam. What the government is primarily looking for are communicable diseases or evidence of drug addiction and abuse.
If you’re a tobacco smoker with lung problems, that would not be something that would make you inadmissible to the U.S. However, if you are found to have a communicable disease, you will face challenges in the U.S visa process because this is a reason for inadmissibility. For more information, see the FAQ page on the U.S. CDC’s website. Also, even though you are examined for evidence of illegal drug abuse, a drug test is not administered in most cases. As of now, they would be looking at physical evidence such as needle marks, missing teeth, and other physical symptoms.