Smoking Linked to Increased Depression Risk

Deborah Brauser

June 09, 2010

June 9, 2010 ( Updated June 11, 2010 ) — Individuals who smoke cigarettes may increase their risk of developing depression, according to results from a large longitudinal study from New Zealand researchers.

The study also found that those who were dependent on nicotine were more than twice as likely to have depressive symptoms as those who were not dependent.

Lead researcher David M. Fergusson, PhD, professor and director of the Christchurch Health and Development Study at the University of Otago, Dunedin, New Zealand, said in a statement that the reason for the relationship between smoking and depression is unclear.

"However, it's possible that nicotine causes changes to neurotransmitter activity in the brain, leading to an increased risk of depression," said Dr. Fergusson.

"The main message of the paper is that in addition to the harm that smoking does to physical health, nicotine dependence may also increase risks of mental health problems," he told Medscape Psychiatry. "The message for clinicians, clearly, is to encourage smoking cessation amongst patients subject to recurrent depressive episodes."

The study was published June 1 in the British Journal of Psychiatry.

Pathways Unclear

Although there have been a number of recent studies that focused on the association between substance use and mental disorders, past research "has not elucidated the pathways by which smoking is associated with depression," write the study authors.

"Does [this association] reflect common factors [from] both smoking and depression? Or does depression increase rates of smoking? Or does smoking lead to depression? The aim of this study was to cast further light on an issue that had been debated for some years," said Dr. Fergusson.

The investigators examined data on 1265 people (635 men, 630 women), born in mid-1977, from the Christchurch Health and Development Study. The cohort participants were asked at the ages of 18, 21, and 25 years whether they had any symptoms of depression during the last 12 months, using the Composite International Diagnostic Interview, and about their smoking habits during the past month, using Diagnostic and Statistical Manual of Mental Disorders, 4th Edition, symptom criteria.

The researchers then used fixed-effects regression modeling to adjust for confounding during the first stage of the study and methods of structural equation modeling for the second stage, which explored the direction of causality.

Significant Association

At the end of the study, the investigators found that "at each age, increasing levels of nicotine-dependence symptoms were significantly associated with increasing rates of depressive symptoms (P < 0.0001)."

In addition, "those reporting at least 5 symptoms of nicotine dependence had rates of depressive symptoms that were 2.13 times (95% confidence interval, 1.98-2.31) those of individuals who reported no symptoms of nicotine dependence."

After adjusting for confounding factors, significant associations were still found between nicotine dependence and symptoms of depression (P < .05).

The researchers also found that although the "nicotine-dependence symptoms were significantly related to depressive symptoms, depressive symptoms were not significantly related to nicotine-dependence symptoms (P = 0.21)."

This means that smoking increased the risk of developing depressive symptoms, not that smoking was increased as a result of the participants being depressed, explain the study authors.

They caution, however, that their study does not prove that smoking causes depression and that it "should be viewed as suggestive rather than definitive."

Dr. Fergusson reported that the next step "is clearly further brain research examining the possible pathways that may link nicotine dependence and mood states."

"It will also be of interest to conduct randomized trials of smoking cessation in depressed patients to determine the extent to which [it] may reduce risks of further depressive episodes," he added.

Important Contribution to the Field

"I liked this study. I think it's an important contribution to the field and really signals the need for more research in this area," Douglas Ziedonis, MD, MPH, professor and chair of the Department of Psychiatry at the University of Massachusetts Medical School and UMAss Memorial Healthcare System in Worcester, told Medscape Psychiatry.

"Several studies have looked at the association between depression and smoking. The new and important contribution from this study was to actually provide some data to support the causality and the link that many of us thought was probably there," added Dr. Ziedonis, who was not involved with this trial.

He noted that he would have liked to have also seen data on a younger age group, especially between the ages of 12 and 16 years. "I don't know how it is in New Zealand, but here in America, tobacco addiction is an adolescent disease. That's when kids start, get hooked, and become dependent for the most part."

"I think this kind of study justifies the [National Institute of Mental Health] and [National Institute on Drug Abuse] to support research that would look at this association in a prospective way with a range of methods, including epidemiological and statistical like with this one, but also looking at brain imaging, genetics, and other ways to look at this causality issue."

"This is a major public health issue, and I think it's of particular interest for psychiatrists to know about," concluded Dr. Ziedonis.

This study was supported by grants from the Health Research Council of New Zealand, the National Child Health Research Foundation, the Canterbury Medical Research Foundation, and the New Zealand Lottery Grants Board. Dr. Fergusson and and Dr. Ziedonis have disclosed no relevant financial relationships.

Br J Psychiatry. 2010;196:440-446. Abstract

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