A single dose of many addictive drugs will produce a protein, called ?FosB (pronounced “delta Fos B”), that accumulates in the neurons. Each time the drug is used, more ?FosB accumulates, until it throws a genetic switch, affecting which genes are turned on or off. Flipping this switch causes changes that persist long after the drug is stopped, leading to irreversible damage to the brain’s dopamine system and rendering the animal far more prone to addiction. Nondrug addictions, such as running and sucrose drinking, also lead to the accumulation of ?FosB and the same permanent changes in the
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