It is my understanding as a non-attorney, non-spokesperson that in Mississippi (my home state) a misdemeanor drug offense will not interfere with getting a permit to carry a concealed firearm. It is spelled out fairly specifically in state law; i would start looking for your state's law on the subject. IIRC, a pattern of regular violations of that sort would be a problem for the carry permit process.
As far as the federal form 4473, there is a question about illegal substance and being an "unlawful user" or an "addict" to them. It appears to this non-attorney, non-spokesperson (not for this forum or any organization) that the form is asking about your current status as a user or addict, but that is my own personal interpretation. I consider that an unlawful restriction of my 2nd Amendment right, but i am almost certainly in the minority.