Since the late 1980s, a popular theory has claimed that increasing levels of the signaling molecule serotonin is an important part of treating depression. This is typified by the antidepressant Prozac, which works by boosting serotonin levels.

When Prozac was launched in the 1980s, it became a popular treatment for depression very quickly. Because of the widespread use of Prozac, the theory that low levels of serotonin caused depression also rose in popularity. But is there sound evidence to support this theory? Many experts think not.

Serotonin deficiency as cause of depression – is it a myth? – Medical News Today.