While some smokers report short-term benefits to symptoms that can be associated with mental illness studies that have measured symptoms such as anxiety, depression, stress, and quality life before and after smoking cessation find that cigarette use is associated with worsened mental health. It may be that some of the benefits a person perceives comes from smoking may actually be due to the elimination of nicotine withdrawal symptoms. Common symptoms of withdrawal include: irritability, restlessness, insomnia, anxiety, depression, increased appetite, and poor concentration. The table below demonstrates the effect smoking cessation has on symptoms of mental health as well as the increases to psychological quality of life and positive affect. The positive effect from stopping smoking is measured as similar to the effect experienced from common anti-depressants!
Taylor G, McNeill A, Girling A, Farley A, Lindson-Hawley N, Aveyard P. Change in mental health after smoking cessation: systematic review and meta-analysis. BMJ. 2014;348:g1151.