Major depression is a mood disorder that causes those who suffer from it to experience a consistently sad, low mood. People with depression are unable to experience pleasure. They lose interest in activities they once enjoyed. They may sleep too much or not enough, and their eating habits may change. At its most severe, depression may lead to suicidal thoughts or actions. Researchers have linked depression to low levels of two chemicals: serotonin and norepinephrine.
Serotonin is a neurotransmitter that promotes feelings of calmness and happiness and reduces feelings of stress, sadness and anger. Serotonin has also been linked with regulating eating and reducing obsessive thoughts and behaviors. Many scientists believe that serotonin helps balance mood. Serotonin is manufactured in the brain and in the intestines. People who are depressed tend to have low levels of serotonin, but it is not clear whether the low levels cause the depression or are caused by the depression.
Norepinephrine is both a hormone and a neurotransmitter. As a hormone, norepinephrine underlies the fight or flight response people experience in dangerous situations. An increase in norepinephrine raises the heart rate, releases glucose into the system and increases the flow of blood to the muscles. A decrease in norepinephrine, on the other hand, lowers levels of arousal and alertness. Low levels of norepinephrine appear to be a factor in depression in some people but not in others.
Based on the above information, it stands to reason that antidepressant medication focuses on increasing the levels or serotonin, norepinephrine or both. The most frequently prescribed antidepressants are SSRIs - selective serotonin reuptake inhibitors - which increase the amount of serotonin in the brain. Atypical antidepressants such as Cymbalta and Remeron increase the levels of both norepinephrine and serotonin. There are two older classes of drugs, tricyclics and monoamine oxidase inhibitors, or MAOIs, that also increase the levels of serotonin and norepinephrine. While effective in treating the symptoms of depression, these older drugs have largely been abandoned due to the side effects they causes. MAOIs also tend to cause severe reactions when combined with other medications or certain types of food such as cheese or chocolate.
Because each medication works on serotonin and norepinephrine in slightly different ways, a person experiencing depression may have to try several different medications before finding the one that works best for him or her. It usually takes between two and three weeks before antidepressant medication reaches its full therapeutic benefit.