Depression, a state of low mood and aversion to activity, can affect a person’s thoughts, behavior, tendencies, feelings, and sense of well-being. A depressed mood is a normal temporary reaction to life events, such as the loss of a loved one. It is also a symptom of some physical diseases and a side effect of some drugs and medical treatments. Depressed mood may also be a symptom of some mood disorders such as major depressive disorder or dysthymia.