Low serotonin levels are believed to be the cause of many cases of mild to severe depression which can lead to symptoms such as anxiety, apathy, fear, feelings of worthlessness, insomnia and fatigue.  The most concrete evidence for the connection between serotonin and depression is the decreased concentrations of serotonin metabolites in the cerebrospinal fluid and brain tissues of depressed people.

If depression arises as a result of a serotonin deficiency then pharmaceutical agents that increase the amount of serotonin in the brain should be helpful in treating depressed patients.  Anti-depressant medications increase serotonin levels at the synapse by blocking the reuptake of serotonin into the presynaptic cell.  Anti-depressants are one of the most highly prescribed medications despite the serious side-effects they can cause.

If depression is mild enough it can sometimes be managed without prescribed medications.  The most effective way of raising serotonin levels is with vigorous exercise.  Studies have shown that serotonin levels are increased with increased activity and the production of serotonin is increased for some days after the activity.  This is the safest way of increasing serotonin levels and many other benefits result from regular exercise.  

Serotonin levels can also be controlled through the diet.  A diet deficient in omega-3 fatty acids may lower brain levels of serotonin and cause depression.  Complex carbohydrates raise the level of tryptophan in the brain resulting in a calming effect.  Vitamin C is also required for the conversion of tryptophan into serotonin.