Depression isn't just feeling down. It's a real illness with real causes. Depression can be set off by stressful life events, like divorce or a death in the family or it can appear suddenly, for no apparent reason. Some people think you can try to get yourself out of a depression. That's not true. Many doctors believe that one thing that may cause depression is an imbalance of serotonin(血清素)—chemical in your body. If this happens, you may have trouble sleeping. Feel unusually sad or irritable. Find it hard to concentrate. Lose your appetite. Lack energy or have trouble feeling pleasure. These are some of the symptoms(症状) that can point to depression—especially if they last for more than a couple of weeks. To help fight depression, the medicine doctors prescribe now most often is Prozac. Prozac isn't a 'happy pill'. It won't turn you into a different person. Some people do experience mild side effects, like upset stomach, headaches, difficulty sleeping, anxiety and nervousness. These tend to go away within a few weeks of starting treatment, and usually aren't serious enough to make most people stop taking it. However, if you are concerned about a side effect, or if you develop a rash(皮疹), tell your doctor right away. And don't forget to tell your doctor about many other medicines you are taking. Prozac has been carefully studied for nearly 10 years. But remember, Prozac is a prescription medicine, and it isn't right for everyone. Only your doctor can decide if Prozac is right for you. Prozac has been prescribed for more than 17 million Americans. Some people think depression is ______.