Exercise is as effective as a second medication for half of depressed patients whose conditions have not been cured by a single antidepressant medication, according to a new study.

Scientists found that both moderate and intense levels of daily exercise can work as well as administration of second antidepressant drug, which is often the case when the antidepressant drug does not fully make patients well.

The study was published in the Journal of Clinical Psychiatry, University of Texas Southwestern medical center.

The four year project is one of the first controlled investigations in the United States to suggest that adding a regular exercise routine, combined with targeted medications, can actually fully relieve the symptoms of major depressive disorder.

"Many people who start on an antidepressant medication feel better after they begin treatment, but they still don't feel completely well or as good as they did before they became depressed," said Dr. Madhukar Trivedi, professor of psychiatry at UT Southwestern and the study's lead author. "This study shows that exercise can be as effective as adding another medication. Many people would rather use exercise than add another drug, particularly as exercise has a proven positive effect on a person's overall health and well-being."

Study participants diagnosed with depression, with ages ranging from 18-70 were divided into groups, each receiving a different level of exercise intensity for 12 weeks. Sessions were supervised by trained staff.

Participants whose average depression length was seven years exercised on treadmills, cycle ergometers or both, kept an online diary of frequency and length of sessions, and wore a heart-rate monitor while exercising at home. They also met a psychiatrist during the study.

Almost 30 percent of patients in both groups achieved full remission from their depression, and another 20 percent significantly displayed improvement, based on standardized psychiatric measurement.

Moderate exercise was more effective in women with a family history of mental illness. A high rate of exercise was more effective for men.

"This is an important result in that we found that the type of exercise that is needed depends on specific characteristics of the patient, illustrating that treatments may need to be tailored to the individual," said Dr. Trivedi, director of the Mood Disorders Research Program and Clinic at UT Southwestern. "It also points to a new direction in trying to determine factors that tell us which treatment may be the most effective."