Lexapro is prescribed for major depression persistently low mood that interferes with daily functioning. To be considered major, depression must occur nearly every day for at least two weeks, and must include at least five of the following symptoms: low mood, loss of interest in usual activities, significant change in weight or appetite, change in sleep patterns, agitation or lethargy, fatigue, feelings of guilt or worthlessness, slowed thinking or lack of concentration, and thoughts of suicide.

Lexapro is also prescribed for generalized anxiety disorder, a condition marked by excessive worry and anxiety that is hard to control and interferes with daily life. To be diagnosed with this disorder, your symptoms must have lasted at least 6 months and you must have at least three of the following: restlessness, fatigue, poor concentration, irritability, muscle tension, and sleep disturbances.

Lexapro works by boosting levels of serotonin, one of the chief chemical messengers in the brain. The drug is a close chemical cousin of the antidepressant medication citalopram. Other antidepressants that work by raising serotonin levels include fluoxetine, paroxetine, and sertraline.