TUESDAY, April 26 (HealthDay News) -- Antidepressants may not improve all symptoms of depression, according to a new study.

Researchers analyzed data from a U.S. National Institute of Mental Health-sponsored study of depression treatment, which involved more than 4,000 people with major depression around the country and is the largest study on depression treatment to date.

While antidepressant medications worked in general, all patients taking them reported three to 13 residual symptoms, and 75 percent reported five or more residual symptoms.

These symptoms included insomnia that occurs in the middle of the night (79 percent); sadness (nearly 71 percent), and problems with concentration and decision-making (nearly 70 percent), said the researchers at UT Southwestern Medical Center in Dallas.

"Widely used antidepressant medications, while working overall, missed these symptoms. If patients have persistent residual symptoms, these individuals have a high probability of incomplete recovery," lead author Dr. Shawn McClintock, an assistant professor of psychiatry, said in a medical center news release.

However, suicidal thoughts discontinued in nearly all cases, the release said.

McClintock said it will be important to develop antidepressants that decrease the other depressive symptoms, and to explore the link between depression and concentration.

The study appears in the April issue of the Journal of Clinical Psychopharmacology.

Depression, which affects about 19 million adults in the United States a year, can increase the risk of heart disease, diabetes, asthma and obesity.

More information

The U.S. Agency for Healthcare Research and Quality has more about antidepressants.