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Why is smoking so difficult to give up? Inhaling a puff of smoke from a cigarette sends nicotine molecules zooming up into the brain within seconds. Then the nicotine grabs hold of receptors on brain cells and releases dopamine, the brain's feel-good chemical, bringing feelings of pleasure and comfort.
Besides dopamine, smoking also activates the release of other molecules in the brain that make you feel good, including chemicals called endogenous opioids, according to 2004 research from the University of Michigan.
And cigarette companies haven't been making it any easier for people to kick the habit. Over the past decade, companies have made changes to the design and ingredients in cigarettes to make them more alluring to first-time smokers, and more addictive for long-time smokers.
These changes include adding ammonia to the tobacco, which converts nicotine into a form that gets to the brain faster, as well as adding holes to cigarette filters that allow people to inhale smoke more deeply into their lungs. Other additions, such as sugar and "moisture enhancers," reduce the dry, burning sensation of smoking, making it a more pleasant experience - especially for new cigarette smokers.
When you stop smoking, and your nicotine receptors stop being activated. Essentially, you're not getting as much dopamine as you're used to, which can cause feelings of withdrawal. In fact, a 2002 study in the Journal of Consulting and Clinical Psychology showed that people who had quit cold turkey had lingering feelings of depression and tension 31 days later.
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为什么戒烟如此困难？吸一口烟可将尼古丁在几秒钟内送入大脑。随后尼古丁将控制脑细胞受体并释放出多巴胺——大脑中感觉良好的化学成分，能给人带来愉悦和舒适的感觉。
密歇根大学2004年的一份研究表明，除了多巴胺，吸烟还能激发大脑中使人感觉良好的其他化学分子的释放，包括被称为内源性类鸦片活性肽的化学分子。
此外，烟草公司的“努力”也致使人们更难戒掉这个习惯。在过去10年，烟草公司改变了香烟的设计和成分，使它们对第一次吸烟的人更有吸引力，使长期吸烟的人更容易上瘾。
这些改变如，在烟草中加入氨，它将尼古丁转变为一种可更快进入大脑的分子形式，并增加了烟嘴中的孔，这使人们能将烟雾更深度地吸入肺部。其他的添加成分如，糖和水分保持剂，这样就减少了吸烟时的干燥和焦灼感，使吸烟成为更愉悦的经历——尤其是对新烟民而言。
停止吸烟后，你尼古丁受体不再被激发，因而你无法得到你习惯得到的那么多多巴胺。这会引起退缩的感觉。实际上，《咨询与临床心理学》杂志2002年刊载的一项研究显示，那些停止使用毒品的人在31天后会有抑郁和紧张的感觉。
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