The little I know,
I did not do extensive study of the medical research. I browsed through various studies available for free online. I saw no source that even remotely suggested that smoking a single cigarette was dangerous in any meaningful sense (i.e. more dangerous than driving a car). The research I saw showed that the danger dropped dramatically as the smoking decreased. I could not find any study showing harmful effects for those smoking less than one cigarette per day.
What possible issur is there in smoking a single cigarette?