Nicotine is interesting because it has different effects at different dosages and metabolizes fast so smokers use it to kind of manually control certain hormonal symptoms. Another example: it's often used as an appetite suppressant because it's a stimulant but many addicts need nicotine after a meal to increase gut motility and stomach acid production or they'll get indigestion. They'll change how they take drags based on the dose needed. It's a drug that both acts on the sympathetic and parasympathetic systems.