Nicotine. The percentage of users who become addicted to cigarettes is 80%, compared to heroin's 35%:
The reason is that the cigarette is the Galil assault rifle of the nicotine delivery world: fast and reliable. Consider that while a heroin user injects a hit and feels a potent euphoric rush about fifteen seconds later, he is not going to inject again for many hours. The cigarette smoker, on the other hand, will typically take ten puffs from a single cigarette and will often smoke many cigarettes in the course of a day. Each puff will deliver nicotine to the pleasure circuit about fifteen seconds later, approximately the same delay as for intravenous heroin. So while a typical heroin addict may get two strong, rapidly delivered hits per day, the pack-a-day cigarette smoker will get two hundred weak, rapidly delivered hits per day.
The Dish 