A Morrisville drug company reported Friday that it received regulatory approval to sell a prescription colon cleanser used to prepare patients for a colonoscopy, a cancer screening recommended for people 50 and older.

The approval comes after consumer advocates and a recent study raised questions about whether similar drugs are safe.

Public Citizen, the nonprofit advocacy group founded by Ralph Nader, recently included sodium phosphate colon cleansers on its online list of dangerous pills and recommended patients use alternative products. Public Citizen's recommendation was based on a study published in November in a medical journal showing that the products may be linked to kidney damage.

Salix Pharmaceuticals, the Morrisville company that won approval for OsmoPrep pills, acknowledges sodium phosphate products carry risks, especially for people with reduced kidney function. The company, which specializes in gastrointestinal treatments, already has a product similar to OsmoPrep on the market, Visicol.

Sodium phosphate products come in solutions and pills and have the effect of potent laxatives. To prepare for a colonoscopy, patients drink about a gallon of the solution or take 40 Visicol pills the size of multivitamins over several hours.

Salix reports that patients only have to take 32 OsmoPrep pills, which are smaller and coated for easier swallowing. Unlike Visicol, OsmoPrep leaves no residue in the colon.

The drugs' package inserts include warnings for physicians and patients to exercise caution, said Doug Bettenhausen, a pharmacist and Salix's executive director for medical affairs.

"You lose lots of fluids with sodium phosphate," Bettenhausen said. "You have to replace these fluids rapidly. If you don't do that, you have kidney issues."

To prevent adverse kidney reactions, patients are advised to drink lots of water after swallowing sodium phosphate products.

New York pathologists examining 7,349 samples of kidney tissue found 20 that came from patients who suffered kidney damage after taking sodium phosphate, according to a study in the Journal of the American Society of Nephrology published in November.

The 20 samples were from patients who had taken sodium phosphate solution, an over-the-counter product that makes up about 85 percent of the market for colon-cleansing products. One patient had taken Visicol, Salix's prescription sodium phosphate pill. OsmoPrep also contains sodium phosphate but less than Visicol.

"The FDA is aware of ... [the New York study] and approved OsmoPrep," said Adam Derbyshire, Salix's chief financial officer.

An FDA spokeswoman said she could not comment.

Despite the FDA approval announced Friday, Salix shares fell $1.51 to $15.08. The stock has fallen 33 percent since October, when it closed at the highest level in the past year.

Bettenhausen said the company is monitoring any adverse events in patients taking the drugs. One patient experienced an adverse reaction out of about 1.8 million patients who have taken Visicol. "That's something extremely rare," he said.

Also, the patient identified by the New York study had reduced kidney function before taking Visicol, he added. "He was not an ideal patient to begin with."

Visicol's package insert warns physicians to be cautious in prescribing the product for patients with impaired kidneys.

Colon cleansers generated about $185 million in sales last year. According to Salix, Visicol's sales were $21 million, or about 11 percent of the total market.

"But we believe it's a growing market," Derbyshire said.

Colonoscopies are one of the colorectal cancer screens recommended for people older than 50, but only half the people in that growing age group have had the cancer screens, according to Salix.