Recent data suggest that, in the United States, the rates of daily smoking and the number of cigarettes smoked per day are declining, suggesting a reduction in population-wide dependence among current smokers. However, there are different groups of people who are more likely to smoke than the average population, such as those with low education or low socio-economic status and those with mental illness. There is also evidence that among smokers, some subgroups may be more dependent than other groups. Men smoke at higher rates than do women and score higher on dependence indices; however, women may be less likely to be successful in quitting, suggesting that women may be more dependent by that criterion. There is an increased frequency of nicotine dependence in people with anxiety disorders. 6% of smokers who want to quit smoking each year are successful at quitting. Nicotine withdrawal is the main factor hindering smoking cessation. A 2010 World Health Organization report states, "Greater nicotine dependence has been shown to be associated with lower motivation to quit, difficulty in trying to quit, and failure to quit, as well as with smoking the first cigarette earlier in the day and smoking more cigarettes per day." E-cigarettes may result in starting nicotine dependence again. Greater nicotine dependence may result from dual use of traditional cigarettes and e-cigarettes. Like tobacco companies did in the last century, there is a possibility that e-cigarettes could result in a new form of dependency on nicotine across the world.
Repeated exposure to nicotine can cause an increase in the number of nicotinic receptors, which is believed to be a result of receptor desensitization and subsequent receptor upregulation. This upregulation or increase in the number of nicotinic receptors significantly alters the functioning of the brain reward system. With constant use of nicotine, tolerance occurs at least partially as a result of the development of new nicotinic acetylcholine receptors in the brain. After several months of nicotine abstinence, the number of receptors go back to normal. Nicotine also stimulates nicotinic acetylcholine receptors in the adrenal medulla, resulting in increased levels of adrenaline and beta-endorphin. Its physiological effects stem from the stimulation of nicotinic acetylcholine receptors, which are located throughout the central and peripheral nervous systems. Chronic nicotinic acetylcholine receptor activation from repeated nicotine exposure can induce strong effects on the brain, including changes in the brains physiology, that result from the stimulation of regions of the brain associated with reward, pleasure, and anxiety. These complex effects of nicotine on the brain are still not well understood.When these receptors are not occupied by nicotine, they are believed to produce withdrawal symptoms. These symptoms can include cravings for nicotine, anger, irritability, anxiety, depression, impatience, trouble sleeping, restlessness, hunger, weight gain, and difficulty concentrating.Neuroplasticity within the brains reward system occurs as a result of long-term nicotine use, leading to nicotine dependence. There are genetic risk factors for developing dependence.