Synergy shares soared as much as 25 percent to $6.58 on Wednesday, before losing some gains to trade at $5.88 at 1115 ET on the Nasdaq. Ironwood shares were up 4 percent at $11.53.

Initial data from a 12-week study on 951 patients showed that more patients on Synergy's drug plecanatide had a complete emptying of bowels, compared with those on a placebo.

The highest dosage of the drug also met the study's secondary goal of showing a higher incidence of diarrhea compared with a placebo.

"In our view, (Synergy's) trial result clearly positions plecanatide as a real competitor in the gastrointestinal tract disorders arena and establishes Synergy as a force to be reckoned with," Raghuram Selvaraju of Aegis Capital Corp said in a note.

Ironwood's drug Linzess, which was approved by the U.S. Food and Drug Administration last August to treat chronic idiopathic constipation and irritable bowel syndrome with constipation in adults, became available in pharmacies last month.

"I originally had expectations that Ironwood's drug is going to probably do about $1.5 billion in (annual) sales in the U.S. and Europe, and I would expect Synergy's drug to easily be able to get over $2 billion," Selvaraju said.

Chronic idiopathic constipation is the most common digestive complaint in the world and affects about 15 percent of people in the United States, according to Synergy.

Most analysts recommended buying Synergy's stock, with some saying Synergy's drug proved as effective as Linzess had in a late-stage study, but caused comparatively lesser diarrhea.

Analyst Scott Henry of Roth Capital Partners said there was a 55 percent chance of Synergy's drug being approved, while Brean Capital's Jonathan Aschoff said he expects the drug to be approved in the United States in 2015.

Synergy is also studying the drug as a treatment for irritable bowel syndrome. Analysts Selvaraju and Henry said they expect positive results from those studies.

(Reporting by Zeba Siddiqui in Bangalore; Editing by Roshni Menon)

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