I was raised to believe that Nicotine is “the most addictive substance on earth”. Maybe the meme flowed through this 1987 New York Times article: Nicotine: Harder to Kick … than Heroin. It was reinforced by authoritative quotes like “Nicotine is the most addictive chemical we know”, or fact sheets indicating that it is 5-10x more potent than cocaine or morphine. My mother’s memorable tales of being a teenage tobacco addict certainly didn’t hurt.
So I was surprised to read on Wikipedia that
Nicotine, a substance frequently implicated in tobacco addiction, is not significantly addictive when administered alone. The addictive potential manifests itself after co-administration of an MAOI, which specifically causes sensitization of the locomotor response in rats, a measure of addictive potential. This may be reflected in the difficulty of smoking cessation, as tobacco contains a naturally-occurring MAOI in addition to the nicotine.
These studies seem to be somewhat controversial; later papers like this one urge caution:
Several recent reports suggest that other chemical substances inhaled along with nicotine in tobacco smoke may play a role in sustaining smoking behavior. However, conflicting results have been obtained with mice and rats and these findings have not yet been validated in nonhuman primates or human subjects.
Of course, if what you really care about is the addictiveness of tobacco, then this whole debate is perfectly irrelevant … but I think it’s fascinating that the role of nicotine is still controversial.