People who suffer from a common form of depression apparently can prevent agonizing relapses by continuing to take their medication even when they feel well, researchers reported Friday.

The findings offer relief to millions of people who suffer from a major form of depression and indicate more research should be done to evaluate similar approaches for other drugs and other forms of depression, experts said."This breakthrough in a common and devastating form of depression offers hope for millions of people who are tormented by recurrent depressive illness," said Alan Leshner, acting director of the National Institute of Mental Health.

In the new study, researchers at the University of Pittsburgh evaluated the drug imipramine, which has long been used to treat a type of mental illness called recurrent unipolar depression.

About half of the estimated 5.4 million Americans who suffer from depression have recurrent unipolar depression. Victims experience severe episodes of symptoms including fatigue, sadness and hopelessness that last for several weeks or months.

Victims usually can get relief by taking an anti-depressant drug such as imipramine. But doctors usually curtail the drugs after the symptoms abate. The devastating symptoms usually recur, as often as every six months.

In the new study, published in the Archives of General Psychiatry, the researchers tested various approaches of using imipramine on 128 recurrent unipolar depression patients age 21 to 65 who had been having bouts every two years for 12 1/2 years on average.

All the patients took 200 milligrams of imipramine a day, which caused a remission of symptoms.

After the patients were stabilized, 28 patients continued to take imipramine at the same dose, 23 patients took only a useless substitute, 26 patients continued only therapy, 26 received therapy and the placebo and 25 received therapy and imipramine.

About 80 percent of those who continued to take the full dose of the drug avoided a relapse for the three years of the study, the researchers said.