Several studies have shown that major depression is more than twice as common in smokers as nonsmokers [source: Mansnerus]. And if one guy is always standing outside smoking, you might want to ask him if he needs to talk -- these studies also indicate that depression is much more likely when a person feels the need to light up a cigarette continuously [source: Mansnerus].
But is a smoker depressed because of the clinging smell of smoke and their yellow teeth? Or are these the unfortunate side effects of the one thing that makes a depressed person happy? Scientists hypothesize that any addiction may be an attempt to self-medicate the depression -- witness a person who drinks to numb pain and drown out negative thoughts -- but the evidence seems particularly compelling in the case of cigarettes. Depression is tied to imbalances of certain chemicals in the brain, most notably the neurotransmitters serotonin and dopamine. Each puff of nicotine stimulates the release of those very neurotransmitters. Even in nonsmokers, a nicotine patch was found to decrease the symptoms of depression [source: Duke University Medical Center].
But not every depressed person turns to cigarettes; some hit the corner bar nightly to fix their brain chemistry. Alcoholism has been linked to a deficiency in serotonin levels in the brain, which can cause depression but also hinder a person's ability to rein in impulses [source: Goleman]. This means that alcoholics may continue to hit the bottle even though they know it's not a long-term solution, but also as a form of self-medication for other areas of life. If these people lack impulse control when it comes to anger, for example, they may turn to more alcohol to feel relaxed and refrain from outbursts.
However, one reason why it's hard to know whether the addiction leads to depression is because of the role of dopamine in addiction. Dopamine is associated with the brain's pleasure and reward center, and while depressed people may lack it, the brain scans of many addicts show that they actually have high levels of dopamine [source: Phillips]. This seems to suggest that addicts' brains start off on a pleasure high that they have to keep feeding, rather than a deficit that they try to address through addiction.
So while comorbidity of addiction and depression is common, it's probably too easy to say that one leads to another. There are a host of other factors involved, including genetics, environment and income level. Gender may also play an important role; it seems that men are more likely to develop the addiction before the depression, whereas women typically experience depression first [source: Albrecht, Herrick].
It's important to remember that even if depression made a person prone to addiction, curing just the depression won't necessarily put an end to the addiction. Once a person is addicted, he or she has two problems to deal with and should consider treatment programs that focus on both issues. Depression may also be a factor in whether someone is able to successfully quit an addiction, especially because quitting a substance or behavior may exacerbate the feelings of sadness.
For more information on depression and addiction, please see the links on the next page.