In 40 minutes, half the effects of nicotine are gone. So smokers get the urge to light up for another dose of the drug. After repeated doses of nicotine, the brain changes. To adjust to too much dopamine, the brain cuts production of the neurotransmitter and reduces the number of some receptors. Now, the smoker needs nicotine just to create normal levels of dopamine in his or her brain. Without nicotine, the smoker feels irritable and depressed. The smoker has trained the limbic system to crave tobacco.

E. Without nicotine, a smoker feels happy and calm.