(Reuters) - Sage Therapeutics Inc said its drug alleviated symptoms of severe postpartum depression, meeting the main goal of a small mid-stage study and sending the company's shares soaring.

About one in seven women experience postpartum depression, a severe form of "baby blues" that eventually interferes with her ability to take care of the baby and handle daily tasks, according to the American Psychological Association.

There are no specific therapies for PPD. Existing options include standard antidepressants and psychotherapy.

Data on 21 patients showed that the drug, SAGE-547, achieved a statistically significant reduction in symptoms at 60 hours, compared to a placebo, on a standard depression scale used in clinical research, Sage said on Tuesday.

Typical antidepressants take about four-to-six weeks to take effect, trial investigator Samantha Meltzer-Brody told Reuters.

"So the rapid onset of response of this drug is unlike anything else available in the field," she said.

A woman with PPD can suffer a whirlwind of emotions, including severe anxiety, panic attacks, thoughts of harming herself or the baby, and feelings of worthlessness, shame, guilt or inadequacy.

Cambridge, Massachusetts-based Sage said it had initiated an expansion of the mid-stage study to determine optimal dosing for the injectable drug, the company's main drug under development.

Sage is also evaluating the drug for use in super refractory status epilepticus (SRSE), a life-threatening seizure disorder, as well as essential tremor.

The drug is expected to generate peak sales of about $610 million by 2021, according to data compiled by Thomson Reuters Cortellis.

The company's shares were up 40 percent at $47.15 in premarket trading.

Hedge fund Kerrisdale Capital said earlier this year that it had shorted the stock, expecting SAGE-547 to fail a critical late stage study involving patients with SRSE.

Data from the study is expected later this year.

(Reporting by Natalie Grover in Bengaluru; Editing by Savio D'Souza and Sriraj Kalluvila)