American Journal of Epidemiology Advance Access originally published online on September 4, 2006
American Journal of Epidemiology 2006 164(9):919-920; doi:10.1093/aje/kwj307
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Letter to the Editor |
TWO OF THE AUTHORS REPLY
Department of Behavioural Sciences in Medicine, Institute of Basic Medical Sciences, Faculty of Medicine, University of Oslo, Blindern, N-0317 Oslo, Norway
(e-mail: ole.klungsoyr{at}medisin.uio.no)
We thank Dr. Sheikh (1
) and Ms. Ross (2) for their comments regarding our recent study assessing the association between cigarette smoking and a subsequent first episode of depression (3).
Dr. Sheikh's (1) major concern is that because of a lack of information on short-lived smoking cessation in our study interviews, we could not distinguish depression caused by smoking cessation from depression caused by smoking. We share Dr. Sheikh's concern about the potential impact of smoking cessation on the risk of depression, and below we elaborate on how this might have influenced our results.
Our definition of smoking cessation between T1 (baseline) and T2 (reinterview) was daily smoking at T1 and no smoking at T2. We found that 132 respondents had quit smoking during follow-up; these participants probably represented the successful quitters. An analysis excluding these persons shows the impact of sustained smoking cessation on the hazard ratios (table 1). There are only minor changes in comparison with inclusion of the quitters, especially for smoking. In the original analysis, we removed the six respondents who had developed depression after quitting smoking, as if no quitters stayed depression-free (worst case). There were 113 respondents who quit smoking during follow-up without developing depression. In the sustained-smoking group, 15 percent developed depression, as compared with 10 percent in the successful-quitting group and 7.8 percent in the sustained never-smoker group. Successful quitters did reduce their risk of depression in comparison with the continuing smokers (cf. Dr. Sheikh's remark).
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What our naive definition of smoking cessation did not capture was those participants who were smoking at both T1 and T2, with one or more unsuccessful attempts to quit during follow-up. In the interviews, there is detailed information on possible decisive causes. The respondents were asked to describe the context of their first depressive episode and when it happened. Then they were asked about specific alternative causes such as physical illness, medication, and use of drugs and alcohol. In addition to the respondent's own opinion, he or she was asked to refer a doctor's statement. None of the 130 respondents who developed depression reported smoking cessation as a possible cause. Stressful life events were reported causes in 80 percent of the depressions. In two thirds of the cases, a doctor's opinion was included, and none of these mentioned smoking cessation either. Smoking cessation programs and unsuccessful attempts to quit are usually of short duration (e.g., 10 weeks (4
)), and depressive symptoms linked to the smoking cessation develop during this period or soon afterwards, in contrast to a potentially smoking-induced depression (we found a significant hazard ratio for 400 or more cigarette-years). Some awareness of smoking cessation as a possible cause of depressive symptoms is expected when these symptoms occur close in time to such a specific intervention and when the smokers with the most symptoms are long-time smokers. At least the doctor should recognize this possibility.
We believe that unsuccessful attempts at smoking cessation played a minor role in our smoking-depression association. Unsuccessful quitters have been shown to have the same risk of depression as successful quitters during the year following treatment for smoking cessation (5). This means that unsuccessful quitting cannot alone account for the difference between the 15 percent incidence of depression in our continued-smoking category and the 10 percent incidence among the successful quitters. In addition, smoking cessation was not mentioned as a possible cause in the interviews; and lastly, many of the reported explanations would have to be disregarded as decisive causes. We are grateful for Dr. Sheikh's setting a focus on relevant information that should be collected routinely with respect to smoking history.
We thank Ms. Ross (2) for her comments and references regarding the multiple possible pathways for smoking to increase the risk of depression. She raises the interesting question of whether certain carcinogens increase the risk of depression through aberrant DNA methylation.
ACKNOWLEDGMENTS
Conflict of interest: none declared.
References
- Sheik K. Re: "Cigarette smoking and incidence of first depressive episode: an 11-year, population-based follow-up study." (Letter). Am J Epidemiol 2006;164:918–19.
[Free Full Text] - Ross CM. Re: "Cigarette smoking and incidence of first depressive episode: an 11-year, population-based follow-up study." (Letter). Am J Epidemiol 2006;164:917–18.
[Free Full Text] - Klungsøyr O, Nygård JF, Sorensen T, et al. Cigarette smoking and incidence of first depressive episode: an 11-year, population-based follow-up study. Am J Epidemiol 2006;163:421–32.
[Abstract/Free Full Text] - Killen JD, Fortmann SP, Schatzberg A, et al. Onset of major depression during treatment for nicotine dependence. Addict Behav 2003;28:461–70.[CrossRef][ISI][Medline]
- Tsoh JY, Humfleet GL, Munoz RF, et al. Development of major depression after treatment for smoking cessation. Am J Psychiatry 2000;157:368–74.
[Abstract/Free Full Text]
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