New research published in two studies suggests that smoking may also affect another vital organ: the brain. In one study, smoking was found to thin the brain cortex in an area suggested to be linked to addiction, meaning long-term smokers could become more prone to addiction the longer they continue to smoke. In the second, successful quitters were found to enjoy the most happiness during periods of abstinence, while a subsequent return to smoking was found to depress mood, suggesting that perceived psychological dependence on smoking as a mood enhancer is in fact quite the reverse.

The cerebral cortex in 22 smokers and 21 never-smokers was measured using Freesurfer, a set of automated tools for reconstruction of the brain’s cortical surface from structural MRI (magnetic resonance imaging) data, and overlay of functional MRI data onto the reconstructed surface. This allowed them to compare cortical differences in the brain between the two sample groups.

The data showed that the smokers were found to have structurally different brains from the never-smokers. The smokers demonstrated cortical thinning in the left medial orbitofrontal cortex (mOFC), an area that is suggested also to be linked to a person's susceptibility to drug addition. These results suggest that the more cigarettes a person smokes, and the longer they smoke, the thinner their cortex and the more susceptible to drug addiction (including nicotine) they become.

"Since the brain region in which we found the smoking-associated thinning has been related to impulse control, reward processing and decision making, this might explain how nicotine addiction comes about," says authors of the study Simone Kühnabd from Ghent Institute for Functional and Metabolic Imaging at Ghent University, Belgium, and Florian Schubert and Jürgen Gallinatd from Clinic for Psychiatry and Psychotherapy at Charité University Medicine, Berlin, Germany. "In a follow-up study, we plan to explore the rehabilitative effects of quitting smoking on the brain."

A second study from researchers at Brown University and the University of Southern California has found that giving up smoking improves mental health as well as physical health. Depression was monitored in five check-ups over a 28 week period in 236 quitters who reported that they were never so happy as during their periods of abstinence, regardless of length. The majority demonstrated one of four main quitting behaviors; non-abstinence, abstinence up until the first check-up, abstinence for only half the study, and total abstinence. Sadly the temporary quitters returned to similar darkened moods, or worse in some cases. “The strong correlation in time between increased happiness and abstinence is a tell-tale sign that the two go hand-in-hand,” said Kahler, who is based at Brown’s Center for Alcohol and Addiction Studies (CAAS).

They recommend that smokers wishing to give up should look forward to the increased mental well-being that comes with quitting, and not fear any perceived psychological sacrifice.

“The assumption has often been that people might smoke because it has antidepressant properties and that if they quit it might unmask a depressive episode,” said Christopher Kahler, corresponding author and research professor of community health at the Warren Alpert Medical School of Brown University. “What’s surprising is that at the time when you measure smokers’ mood, even if they’ve only succeeded for a little while, they are already reporting less symptoms of depression.”

Previously, scientists have found it difficult to prove conclusively that quitting improves peoples' moods, since smokers often rely on nicotine to relieve anxiety and depression, which often spike in the short term after quitting.

The studies were published in the journal Biological Psychiatry and Nicotine & Tobacco Research respectively. The National Institute on Drug Abuse funded the latter study.