For the millions of women who ride an emotional roller coaster the week before their menstrual periods, there is hope of relief: A large Canadian study shows that the antidepressant Prozac effectively reduces symptoms in more than half of women with the most severe form of premenstrual syndrome.

The new findings confirm results from a handful of smaller studies of Prozac (also known by its chemical name, fluoxetine) and provide some of the most striking evidence of the biological roots of premenstrual syndrome (PMS). The study shows that PMS is a "real disorder, that it differs from ordinary depression and that it responds to Prozac," said David Rubinow, clinical director of the National Institute of Mental Health (NIMH).

The study, which appears in today's New England Journal of Medicine, also establishes another use for Prozac, the most widely prescribed antidepressant in the United States. Prozac has been taken by 16 million patients worldwide since its introduction in 1986, according to its manufacturer, Eli Lilly and Co, which paid for the new study. The drug has been approved in recent years for a growing number of disorders, from obsessive-compulsive disorder to bulimia.