Oslo: Smokers are more likely to suffer from depression than non-smokers, researchers in have found.

An 11-year study of adults found that the level of depression increased the more cigarettes smoked and heavy smokers – those on more than 20 a day – were four times more likely to suffer.

The report from the University of Oslo concluded that nicotine may change brain levels of the mood enhancing chemical serotonin in long-term smokers.