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Addiction was originally defined by the appearance of physical symptoms—such as sweating, sniffling, and trembling—when a drug was withdrawn from an addicted person or animal. It was also thought that addiction was accompanied by adaptation, in which more and more of the drug is required to produce the same effect.

So long as the focus was on opium-derived drugs such as morphine and heroin, this definition was appropriate. Beginning in the 1960s, however, scientists realized that it did not define the properties that render cocaine and other drugs so dangerous. Cocaine does not produce adaptation, and its withdrawal does not result in physical symptoms that can be seen in laboratory animals. Cocaine withdrawal does, however, produce an intense depression that disappears when the drug is again available. Similarly, nicotine withdrawal produces psychological symptoms such as restlessness, anxiety, irritability, difficulty in concentrating, and a craving for the drug. Today these psychological withdrawal symptoms are recognized as valid indicators of addiction.

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