Smokers who believe their habit relieves stress and depression are mistaken, according to new research.
Experts found that quitting smoking can be just as effective in tackling depression and anxiety as taking antidepressants.
Writing online in the British Medical Journal (BMJ), researchers said the effect of quitting was the same, if not bigger, than for the tablets.
The team, from the universities of Birmingham, Oxford, and King's College London, analysed 26 studies for their research.
They found quitters experienced a significant drop in anxiety, depression and stress.
The effect was the same among the general population of smokers as those with a diagnosed mental health problem.
"Both psychological quality of life and positive affect significantly increased between baseline and follow-up in quitters compared with continuing smokers," the researchers said.
They concluded: "Smoking cessation is associated with reduced depression, anxiety, and stress and improved positive mood and quality of life compared with continuing to smoke."
Participants in the studies were aged 44 on average, smoked about 20 cigarettes a day, and were followed up for an average of six months.
The researchers said healthcare professionals who have been reluctant to offer stop-smoking advice to people with mental health disorders, for fear of quitting making them worse, should be encouraged by the findings.
They acknowledged many smokers, including those without mental health problems, believe smoking helps them wind down.
They said: "Although most smokers report wanting to quit, many continue as they report that smoking provides them with mental health benefits.
"(Research shows) that regular smokers report smoking cigarettes to alleviate emotional problems and feelings of depression and anxiety, to stabilise mood, and for relaxation as well as relieving stress."
Gemma Taylor, the University of Birmingham researcher who led the study, said: "It is hugely encouraging to be able to demonstrate that smoking cessation leads to an improvement in mental health.
"Smoking rates in the general population have declined substantially over the last 40 years.
"However, the rates of smoking in people with mental health problems have barely changed.
"Part of this disparity is due to the myth that stopping smoking will worsen mental health. I believe this research debunks this myth and I hope that these findings motivate people with and without mental health problems to stop smoking."
Professor Paul Aveyard, from the University of Oxford, said: "Patients often say to me, 'Doctor, I'm too stressed to stop smoking now.' I hope doctors will now reassure those patients that there's a good chance that stopping smoking will make you less stressed.
"In fact, for people with chronic mental health problems, stopping smoking might be an effective treatment."
Sophie Corlett, director of external relations at mental health charity Mind, said: " Shockingly almost half of all cigarettes sold in England every year are smoked by people with mental health problems, so we welcome any research that raises awareness of the damaging impact that smoking can have on mental health as well as physical health.
"People with mental health problems tell us they often smoke to cope when they are feeling unwell, but they then struggle to take steps to stop smoking and find that services that help people to quit do not meet their needs.
"A high proportion of people with mental health problems die earlier due to smoking-related diseases, so it's vital that existing cessation programmes are adapted to take mental health into account."