Health Care Financing Administration US Department of Health and Human Services Kansas City, MO 64106
In a study of young men in the US Army, Miller et al. confirmed an association between cigarette smoking and suicide that was previously found in a cohort of older men. A dose-response relation was demonstrated after controlling for age, alcohol intake, and other social factors (1).
A major limitation of this study was the loss of follow-up of an unknown section of the study cohort. Consequently, the data on suicide were known for only those members of the cohort who died while on active duty. Although the authors acknowledged an incomplete ascertainment of suicides, they dismissed any consequent bias because they had no reason to suspect differential suicide rates among the two sections of the study cohort (1). Depending on the size of the subcohort lost to follow-up, the suicide rates may have been different if active duty was in some way associated with smoking and/or suicide. Such a possibility cannot be ruled out.
The authors (1) failed to discuss a biologic mechanism by which cigarette smoking could have caused or contributed to an increased risk of suicide. They ruled out the smoking-related cancers as a link because of the relatively young age of the study cohort. However, they recognized the fact that their study was missing data on mental illness and other possible risk factors, such as homosexuality and post-traumatic stress, that are not uncommon in the armed forces and are known to be associated with suicide. There is no evidence to characterize the neurochemical effects of tobacco smoke as suicidal. On the contrary, smoking has antidepressant properties (2
, 3
). Nicotine in cigarette smoke is known to relieve anxiety and anger in stressful situations (4
6
), it is a stimulant (7
), and it improves cognitive performance and focused attention (8
).
Major depression and other mental illnesses are a major risk factor for suicide in adults of all ages (1, 9
, 10
). Smoking has a rather complex association with depression and related mental disorders. The incidence and prevalence of smoking have been reported to be relatively higher among depressed persons (2
, 11
), and the prevalence of depression is relatively higher among smokers (12
). Cessation of smoking often results in acute depression and anxiety (13
17
), more so among smokers with a history of depression (3
, 12
, 13
).
On the one hand, an increased prevalence of smoking in this cohort (1) may have resulted in an increased prevalence of depression, which, in turn, may have led to the increased risk of suicide. On the other hand, a higher prevalence of depression among the smokers may have resulted in an increased risk of suicide. It may also be that some of the smokers succeeded or tried to quit smoking and, as a consequence, developed major depression. In any case, depression and other stress-related mental illnesses, with or without smoking cessation, are critical factors in the link between smoking and suicide. Consequently, any study that does not account for depression and other mental disorders cannot reliably confirm or reject an association between cigarette smoking and suicide.
Editor's note: The views expressed by Dr. Sheikh in this letter to the editor do not represent the views and policies of the Health Care Financing Administration or the US government.
REFERENCES
Harvard School of Public Health Harvard Injury Control Research Center Boston, MA 02115
Social Sectors Development Strategies, Inc. Natick, MA 01760
US Army Research Institute of Environmental Medicine Natick, MA 01760
Dr. Sheikh (1) correctly points out that our analysis (2
) was restricted to those members of the cohort who remained active-duty army personnel and that therefore those soldiers who killed themselves after leaving the army were not available for analysis. However, his statement that we dismissed any consequent bias because we had no reason to suspect differential suicide rates among those available and those lost to follow-up is not accurate. We never claimed that we expected the suicide rate among active-duty soldiers and those who leave the army to be the same. Rather, we said only that there is no a priori reason to expect that the association between smoking and suicide will differ substantively among those who remain and those who stay in the army. This statement seems reasonable, since other published studies among quite different populations have also found a positive association between smoking and suicide (3
8
). Consistent with a smoking-suicide relation among all soldiers is our finding (not published) that hospitalization for depression among all soldiers (i.e., those who left the study and those who remained) was more likely for current than for never smokers. In fact, those men who smoked more than one pack of cigarettes per day were almost twice as likely as never smokers to be hospitalized for depression.
We agree with Dr. Sheikh (1) that depression and other mental illness are critical factors in the link between smoking and suicide. In our paper (2
), we explicitly stated that a limitation of our study was that we lacked information about some risk factors for suicide, including mental illness. We disagree, however, that "any study that does not account for depression and other mental disorders cannot reliably confirm or reject an association between cigarette smoking and suicide" (1
, p. 691). We can and did demonstrate such an association; what we could not determine from our study was whether mental illness mediates or modifies the association between smoking and suicide. Our finding of a dose-response relation between smoking and suicide is consistent with a causal effect (e.g., smoking leading to depression leading to suicide) for which there is, contrary to Dr. Sheikh's assertion, some evidence (9
). Our findings are also consistent with other noncausal hypotheses, such as depression leading both to self-medication with cigarettes and to suicide and with higher cigarette doses being better markers of some unidentified characteristic that predisposes persons to both suicide and smoking. Although the association we found does not imply causality, our findings nevertheless suggest that the smoking-suicide connection is not likely to be explained by the greater tendency of smokers to be White, drink heavily, have less education, or exercise less often.
REFERENCES