These results should be interpreted in light of five limitations: First, the prevalence of suicidal behaviors is low, limiting statistical power. Second, analyses were based on cross-sectional naturalistic data. Neither temporal nor causal priority can be established even though we treated measures of smoking as the predictors and suicidal behaviors as the outcomes. Third, the assessment of smoking excluded consideration of smokeless tobacco use, limiting the generality of results. Fourth, the assessment of mental disorders was based on self-reports rather than clinical assessments. Although blinded clinical reappraisal interviews found good concordance between CIDI diagnoses and clinical diagnoses, some respondents might have consciously and consistently failed to disclose information about mental disorders and suicidality in both the main survey and the clinical reappraisal interviews. Fifth, the ad hoc model-fitting for the smoking variables might have resulted in over-estimating the strength of the associations between smoking and the outcomes. As noted above, though, this was done consciously to investigate dose-response relationships and to create a strong baseline model to evaluate the effects of controlling for mental disorders.
Within the context of these limitations, the documentation of elevated gross cross-sectional ORs between recent smoking and recent suicidal behaviors is consistent with other cross-sectional surveys (Brener et al, 1999
; Frank and Dingle, 1999
; Hallfors et al, 2004
; King et al, 2001
). The finding that current smoking is a more powerful predictor than past smoking is consistent with the small number of previous studies that examined that specification (King et al, 2001
; Miller et al, 2000b
). The finding of a dose-response relationship between quantity of current smoking and suicidal ideation is consistent with the two previous studies that examined that association (King et al, 2001
; Wu et al, 2004
The specifications involving quantity, frequency, history, and recency of dependence go beyond previous research and generally are inconsistent with most plausible hypotheses that implicate smoking as a cause of suicidal behaviors because they do not document a dose-response relationship between smoking and suicidal behaviors. The most important of these specifications are that quantity of cigarette smoking is not related either to suicide plans or attempts, that the ORs for nicotine dependence are very similar for remitted and current cases, and that number of years smoking is unrelated to the outcomes either among current or past smokers.
As noted in the introduction, previous studies have generally failed to explain the associations between smoking and suicidal behavior outcomes with the less detailed measures of mental disorders included in their analyses. We found, in comparison, that these associations can be explained with controls for the more carefully and comprehensively assessed CIDI diagnoses. Although it is not possible to make a clear adjudication between contending causal hypotheses to explain the mechanism involved in this finding, the finding makes it clear that future research on smoking and suicidal behavior needs to focus much more centrally than most previous research on mental disorders either as common causes, markers, or mediators.
Our findings are consistent both with the common cause hypothesis (i.e., that both current smoking and current suicidal behaviors are consequences either of mental and substance use disorders or of more distal causes for which current mental and substance use disorders serve as proxies) and with the mediation hypothesis (i.e., that smoking has a causal effect on suicidal behaviors that is mediated by mental disorders), although absence of a dose-response relationship between quantity-frequency of smoking and the suicidal behavior argues indirectly against the mediation hypothesis.. Prospective cohort studies are needed to resolve the problem of distinguishing between common causes and mediation. This can happen, though, only when a clear temporal order exists between the putative common cause (e.g., mental disorder) and the independent variable (e.g., smoking). It is important to note that this temporal order has never been established in prospective cohort studies of smoking and suicidal behavior. In the Hemmingsson and Kriebel 26-year prospective study briefly described in the introduction, for example, smoking and emotional problems were both measured at baseline, making it impossible to distinguish between a temporal order in which early smoking predicted the subsequent onset of emotional problems that were already in existence as of the baseline assessment, the reverse, or a situation where unmeasured common causes led to both.
Future research needs to be designed in such a way as to distinguish temporal and causal priorities between smoking and mental disorders to resolve this uncertainty. Although based on retrospective data, previously reported analyses in the baseline NCS using retrospective age-of-onset reports illustrate the logic required in future prospective investigations. Those earlier analyses found reciprocal predictive associations of (i) temporally primary regular smoking with subsequent onset of mental disorders (Breslau et al, 2004a
) and (ii) temporally primary mental disorders with subsequent onset of regular smoking (Breslau et al, 2004b
). The evidence for mental disorders predicting subsequent onset of smoking was much stronger, though, than of smoking predicting subsequent onset of mental disorders. This finding is consistent with the interpretation that mental disorders, or some causal factors of which mental disorders are markers, cause both smoking and suicidal behaviors. The ultimate test of this hypothesis, of course, would require an intervention that manipulated current smoking and determined whether this resulted in a reduction in suicide. Although such an experiment would probably be logistically intractable, a practical alternative might be a quasi-experimental evaluation that took advantage of aggregate time-space variation in some policy variable (e.g., size of cigarette tax) that affected smoking behavior for reasons that could plausibly be assumed to be otherwise independent of suicidal behaviors.