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Over 25 years ago, the US Surgeon General’s report, The Health Consequences of Nicotine Addiction, concluded that nicotine was the drug in tobacco that caused addiction.24 Many international medical authorities, including WHO, have confirmed the findings that tobacco products are highly addictive.89 Dependence on nicotine develops quickly. Studies show that non-daily tobacco use triggers the emergence of nicotine dependence – in the second Development and Assessment of Nicotine Dependence in Youth study, where subjects had smoked at least one cigarette, 62% smoked at least once per month, 53% experienced dependence symptoms and 40% experienced escalation to daily smoking.90 Nicotine is the key chemical making smoking addictive, but nicotine is not responsible for the harmful effects of smoking, which are caused mainly by tar, oxidising chemicals, carbon monoxide and other constituents of tobacco smoke.17,24,89
Dependence on smoking is a complex process. It requires a close link in time between the context in which smoking occurs, its rituals, the sensory stimuli of touch, taste and smell, and the extremely rapid delivery of nicotine to the brain that occurs when smoking a modern cigarette. Evidence suggests that psychosocial, biological and genetic factors all play a role in nicotine addiction.24,89,91
Greater understanding of the neurobiology of nicotine dependence has the potential to improve the use of existing cessation therapies and is helping to develop new compounds to aid smoking cessation. When cigarette smoke is inhaled, the large surface area of the lungs means that nicotine is rapidly absorbed into the pulmonary venous circulation and travels quickly to the brain through the bloodstream.92 Nicotine in tobacco smoke reaches the brain reward system within seconds of inhalation.93 This nicotine affects multiple types of nicotine receptors in the brain including, but not confined to, the α4β2 nicotinic acetylcholine receptor. Activation of this and other receptors triggers the release of dopamine and other neurotransmitters. This reward system is the common pathway for the experience of pleasure from many different social, physical and chemical stimulants, including other drugs of addiction such as cocaine and opiates. As well as the activation of the reward system, the negative effects of nicotine withdrawal are important factors in the continuation of smoking.
Genetic factors play a role in the differing patterns of smoking behaviour and smoking cessation. The degree of susceptibility to developing tobacco addiction – as well as the ease or difficulty of quitting and sustaining abstinence – has been reported from twin and adoption studies. This research shows a high degree of heritability of cigarette smoking (50–70%).94,95 The finding points to an understanding of why smokers vary widely in their relationship to tobacco and their ability to quit. Genetic factors have a substantial role in nicotine withdrawal symptoms, cigarette consumption, difficulty quitting and response to smoking cessation therapies.95 However, a useful way to target treatment based on genetics has not yet been shown. The studies also indicate that there may be some smokers who never fully overcome their addiction, or who can never quit all nicotine use.89