Aug 7 (Reuters) - The U.S. drug regulator's approval of Biogen and Sage Therapeutics' first-of-its-kind postpartum depression (PPD) pill is unlikely to allay the drugmakers' growth concerns, analysts said on Monday.

Adverse commentary after their PPD drug was approved by the U.S. Food and Drug Administration (FDA) on Friday sent Sage shares tumbling 40%, while Biogen dropped 3% in premarket trading.

At least four brokerages cut price targets on Biogen, while two downgraded Sage.

Their drug, Zurzuvae, became the first oral treatment for a common complication that impacts one in eight women during and after pregnancy, and affects their ability to return to normal functioning.

The FDA's decision to decline the drug's approval for a much larger cohort of patients with clinical depression, along with strict safety warnings for its use in PPD, is still a "strike against (Biogen's) management", Baird analyst Brian Skorney said.

"We think this is going to be a tough indication to build a big market out of, even at a high premium price," Skorney said in a research note.

There is a major need for PPD drugs as existing treatments include general anti-depressants that take up to six weeks to show effect, Biogen said in February. Zurzuvae improved symptoms of PPD after 15 days in a late-stage trial. Zurzuvae's use as a clinical depression treatment represents a more than $1 billion sales opportunity compared with $250 million to $500 million potential for postpartum depression, said Jefferies analyst Michael Yee.

"Postpartum depression is not necessarily where a big commercial opportunity is," Biogen CEO Christopher Viehbacher said last month.

The U.S. FDA said last week that additional studies might be required to support the drug's approval for major depressive disorder (MDD), or clinical depression.

Biogen is unlikely to "quickly move forward on another late-stage study for MDD", given its focus on saving costs and its recent announcement of job cuts, Yee said.

(Reporting by Mariam Sunny and Bhanvi Satija in Bengaluru, writing by Manas Mishra, editing by Vinay Dwivedi)