Smoking and depression
By Margaret Rogers
The association between smoking and depression has been well established but research to date has 
focused on the ‘causal hypothesis’ rather than ‘shared-vulnerability’ hypothesis. New research suggests that people who smoke heavily are at a three times higher risk for major depression compared to people who have quit their heavy smoking habit.
Salma Khaled Ph.D., said ‘Under the shared-vulnerability hypothesis, ever-heavy smokers may be expected to have similar elevated risk for major depressive episode irrespective of their smoking status during follow-up. Our results point to the contrary.’
Khaled and a team of researchers from the Mental Health Centre for Research and Teaching, evaluated data from 3,824 adults in the Canadian National Population Health Survey. Khaled now works at the University of Calgary.
The survey participants were initially interviewed between 1994 and 1995. They had further interviews every second year until 2006-2007. The participants were required to have maintained their smoking status as either ‘current’, ‘former’, or ‘never smokers’ throughout the entire period of the surveys.
Khaled said ‘Heavy smokers were identified as those who smoked 20 or more cigarettes per day. Ever-heavy smokers (current and former) may share similar genetic, behavioural, and environmental vulnerabilities, at least for heavy smoking initiation’.
Khaled believes that if these factors were wholly to blame for depression, as given by the shared-vulnerability hypothesis, then we would see former heavy smokers and current smokers with an equal likelihood of having a major depressive episode (MDE).
She added ‘However, if the persistence of the exposure (current as opposed to former) had the dominant effect on the risk for MDE, then current heavy smokers would be expected to have higher risks of MDE relative to former heavy smokers.’
The researchers found that over the 12 year period of the survey, the risk of MDE for all the participants was 13.2%. This was made by averaging the different group percentages as follows:
- Among former heavy smokers the risk was 7.1%
- Among those who have never smoked the risk was 12.2%
- Among current heavy smokers the risk was 26.7%
After taking all other factors into account, such as age, sex and stress, the researchers say that these statistics showed a significant hazard ratio of 3-1 for current heavy smokers compared to those who had quit smoking.
The study findings are published in the Journal of Psychiatric Research.
