【成瘾】
成瘾（addiction）是指一种重复性的强迫行为，即使这些行为已知可能造成不良后果的情形下，仍然被持续重复。这种行为可能因中枢神经系统功能失调造成，重复这些行为也可以反过来造成神经功能受损。瘾可用于描述生理依赖或者过度的心理依赖，例如物质依赖，药物滥用（即俗称的滥药、毒瘾）、酒瘾、烟瘾、性瘾。或是持续出现特定行为（赌、暴食），网瘾、赌瘾、官瘾、财迷、工作狂、暴食症、跟踪狂、偷窃狂、整形迷恋、购物狂、发表废文甚至恋物癖等，是生理或者心理上，甚至是同时具备的一种依赖症。
瘾有分为物质成瘾及Behavioral addiction，行为成瘾是和物质无关的强迫症，如赌瘾和网瘾。在这几种通常的用法中，瘾是描述一种某人高频率反复从事可能对其身心健康和社交生活有害的活动的一种强迫行为。而精神疾病诊断与统计手册的第五版DSM-5中有将赌瘾（gambling disorder）列入。有时成瘾（addiction）会和物质依赖（substance dependence）混淆。两者主要的不同是：物质依赖者在中断物质使用后，会出现戒断症状，甚至造成更多的使用该物质，而成瘾是强制性的摄取某种物质或从事特定行为，不一定有戒断症状。
物质成瘾会对个人和社会带来显著的影响，包括成瘾物质带来的直接影响、伴随的医疗费用、长期的并发症（例如吸烟可能造成的肺癌、酒瘾可能会有的肝硬化、静脉注射甲基苯丙胺会出现的meth mouth症状）、神经可塑性（因为经验原因引起大脑结构的改变）带来的影响、以及后续生产力的下降。成瘾的典型现象包括对于物质或是行为的无法控制及过度关注，虽然有不良结果，却仍然继续摄取成瘾物质或从事特定行为的情形。伴随著成瘾的习惯或是行为模式通常是立即性的满足（短期回报）及延迟出现的不良影响（长期不良结果）。
有时在口语上，瘾也用于指某些人的一些癖好，例如读书、收集（集邮）、看电视、玩游戏、购物、工作、上网、运动及进食等。不过在本条目中，瘾主要是被用于滥用药物和物质滥用问题，也就是有生理依赖或者过度心理依赖的行为。
== 医学观点==
医学上，成瘾是脑中犒赏系统在基因转录及表观遗传机制上出现的失调，成瘾有许多心理上的原因，但依生理来说，是在长期暴露在高度的成瘾刺激原（addictive stimulus，例如吗啡、可卡因、性交、赌博等）后出现的情形The strong correlation between chronic drug exposure and ΔFosB provides novel opportunities for targeted therapies in addiction (118), and suggests methods to analyze their efficacy (119). Over the past two decades, research has progressed from identifying ΔFosB induction to investigating its subsequent action (38). It is likely that ΔFosB research will now progress into a new era – the use of ΔFosB as a biomarker. ...ConclusionsΔFosB is an essential transcription factor implicated in the molecular and behavioral pathways of addiction following repeated drug exposure. The formation of ΔFosB in multiple brain regions, and the molecular pathway leading to the formation of AP-1 complexes is well understood. The establishment of a functional purpose for ΔFosB has allowed further determination as to some of the key aspects of its molecular cascades, involving effectors such as GluR2 (87,88), Cdk5 (93) and NFkB (100). Moreover, many of these molecular changes identified are now directly linked to the structural, physiological and behavioral changes observed following chronic drug exposure (60,95,97,102). New frontiers of research investigating the molecular roles of ΔFosB have been opened by epigenetic studies, and recent advances have illustrated the role of ΔFosB acting on DNA and histones, truly as a ‘‘molecular switch’’ (34). As a consequence of our improved understanding of ΔFosB in addiction, it is possible to evaluate the addictive potential of current medications (119), as well as use it as a biomarker for assessing the efficacy of therapeutic interventions (121,122,124). Some of these proposed interventions have limitations (125) or are in their infancy (75). However, it is hoped that some of these preliminary findings may lead to innovative treatments, which are much needed in addiction. Table 1: Summary of plasticity observed following exposure to drug or natural reinforcers date=20210809043020 "。重复暴露在成瘾刺激原是主要导致成瘾以及维持成瘾现象的主要病理因素。成瘾刺激原有二个特性，一个是其正向增强（接触后会增加再去进行类似行为的可能性），另一个是内在犒赏（认为此物质或是行为有趣、会想要再去进行）。
转录因子ΔFosB是各种成瘾（行为成瘾或物质成瘾）发展中的关键成份及共同因素。二十多年针对ΔFosB在成瘾当中的研究，结果指出成瘾的出现以及伴随的强迫行为加剧或减弱，都和伏隔核中D1型中度多刺神经元中ΔFosB的基因过度表现（genetic overexpression）有关。因为ΔFosB基因表现与成瘾之间有因果关系，ΔFosB在preclinical research中常作为成瘾的生物标记The strong correlation between chronic drug exposure and ΔFosB provides novel opportunities for targeted therapies in addiction (118), and suggests methods to analyze their efficacy (119). Over the past two decades, research has progressed from identifying ΔFosB induction to investigating its subsequent action (38). It is likely that ΔFosB research will now progress into a new era – the use of ΔFosB as a biomarker. ...ConclusionsΔFosB is an essential transcription factor implicated in the molecular and behavioral pathways of addiction following repeated drug exposure. The formation of ΔFosB in multiple brain regions, and the molecular pathway leading to the formation of AP-1 complexes is well understood. The establishment of a functional purpose for ΔFosB has allowed further determination as to some of the key aspects of its molecular cascades, involving effectors such as GluR2 (87,88), Cdk5 (93) and NFkB (100). Moreover, many of these molecular changes identified are now directly linked to the structural, physiological and behavioral changes observed following chronic drug exposure (60,95,97,102). New frontiers of research investigating the molecular roles of ΔFosB have been opened by epigenetic studies, and recent advances have illustrated the role of ΔFosB acting on DNA and histones, truly as a ‘‘molecular switch’’ (34). As a consequence of our improved understanding of ΔFosB in addiction, it is possible to evaluate the addictive potential of current medications (119), as well as use it as a biomarker for assessing the efficacy of therapeutic interventions (121,122,124). Some of these proposed interventions have limitations (125) or are in their infancy (75). However, it is hoped that some of these preliminary findings may lead to innovative treatments, which are much needed in addiction.。ΔFosB在这些神经元的表现一方面会直接调高药物Self-administration及犒赏敏感度，也会透过正增强达到这些效果，另一方面也会降低对厌恶（aversion）的敏感度name="ΔFosB behaviors"。
腹侧被盖区被认为与神经生物学理论中的成瘾现象有关。
== 药物成瘾 ==
诊断药物成瘾可诊断为生理成瘾、成瘾有增加或减退迹象、和没有成瘾。DSM-IV中介绍的包括：
*303.90 酒精依赖
*304.00 鸦片依赖
*304.10 镇静剂依赖、催眠药依赖、或抗焦虑药依赖（包括苯二氮平类药物成瘾和巴比妥依赖）
*304.20 可卡因依赖
*304.30 大麻依赖
*304.40 安非他命依赖（或似安非他命类）
*304.50 致幻剂依赖
*304.60 Inhalant依赖
*304.80 多种物质依赖
*304.90 Phencyclidine（或似苯环利定类）依赖
*304.90 其他（或未知）物质依赖
*305.10 尼古丁依赖
==流行病学==
因为文化的不同，特定时间内出现药物成瘾或是行为成瘾的比例（即患病率）会随时代及国家而不同，也会因为年龄层、社会经济地位等人口学资料而不同。澳洲2009年的药物滥用患病率为5.1%。依照美国2011年在青少年中抽样的结果来看，其酒瘾及非法药物滥用的lifetime prevalence（个体从出生后，一直到接受抽样之前，曾出现过的比例）分别是8%及2-3%。