Shares in Raleigh-based Salix Pharmaceuticals (Nasdaq: SLXP) plunged Monday on news that the FDA had requested more information about a constipation drug.

The company also put off plans to hire a big sales force to market the drug had it been approved.

Shares fell more than 12 percent, or $6.94, to close at $46.25. Trading was heavy at some 3.3 million shares. That was four times the daily average.

Salix announced the FDA decision on Friday after the markets closed, saying it didn’t receive the letter until 5 p.m. that day.

Shares also fell in Progenics Pharmaceuticals Inc. It had licensed the constipation medicine Relistor licensed to Salix.

Progenics plunged 44 percent to $6.09 at 9:51 a.m., after declining to $5.94 in its biggest intraday drop since March 2008.

The U.S. Food and Drug Administration asked for more data about the treatment for opioid-induced constipation in adult patients with chronic, non- cancer pain, the companies said in a statement July 27.

Relistor was approved in 2008 for opioid-induced constipation in patients with advanced illnesses who haven’t responded to laxative therapy. Progenics licensed Relistor to Salix worldwide except for Japan in February 2011.

The companies intend to request a meeting with the FDA to better understand why wider use of Relistor was rejected. The meeting may be scheduled in 45 to 60 days, Michael Freeman, a spokesman for Salix, said in an e-mail today.

Salix will no longer add an additional 160 primary-care sales representatives later this year and plans to update its 2012 earnings forecast when it releases second-quarter results on Aug. 8, Freeman said.

The FDA in April extended its review of the application for wider use of Relistor. The agency didn’t ask for new studies at the time, the companies said in a statement at the time.

Opioids relieve pain by working with receptors in the brain and spinal cord and also interact with receptors in the gastrointestinal tract and can cause debilitating constipation, according to Salix. Subcutaneous Relistor injections decrease opioids’ constipating effects without affecting their ability to relieve pain.

(Bloomberg contributed to this report.)