Serotonin is the neurotransmitter most linked to mood disorders like depression, but scientific evidence showing the role it plays is surprisingly slim.
The strongest link between serotonin and depression is probably the fact that the widely used antidepressants known as selective serotonin reuptake inhibitors (SSRIs), such as Prozac and Zoloft, boost brain levels of serotonin. The efficacy of these drugs provides support for the theory that depression is caused by too little serotonin. SSRIs do not effectively treat depression in all people, however, and some newer antidepressants on the market have less of an effect on serotonin levels and are still proving to be effective at treating depression.
While many experts believe that serotonin plays a role in depression, they have not yet reached a consensus on what that role is. It appears that the relationship between serotonin and depression is far more complex than simply low levels. In fact, even though increasing serotonin seems to improve depression in some people, studies have failed to consistently show that depressed people actually do have low levels of serotonin. This has been likened to using aspirin to treat a headache--just because it works doesn't mean that the headache was caused by a lack of aspirin.
So it is still unknown whether a problem with serotonin causes depression, depression causes a problem with serotonin, or some other factor causes both issues. Research is ongoing, and so far scientists are discovering that the relationship between depression and serotonin is not clear-cut.
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