THURSDAY, Dec. 11 (HealthDay News) -- Two prescription drugs used to cleanse the bowel before a colonoscopy may cause kidney damage, the U.S. Food and Drug Administration said Thursday.

The FDA said it would now require a black-box warning on the oral sodium phosphate products Visicol and OsmoPrep, alerting consumers about the risk of acute phosphate nephropathy -- a type of severe kidney injury.

"The FDA has received reports of 20 unique cases of kidney injury associated with the use of OsmoPrep," Dr. Joyce Korvick, deputy director of FDA's Division of Gastroenterology Products at the Center for Drug Evaluation and Research, said during an afternoon teleconference. "Of the reported cases, three were biopsy-proven cases of acute phosphate nephropathy. The onset of kidney injury in these cases varied, occurring in some within several hours of use of these products and in other cases up to 21 days after use."

The agency said it has told the products' manufacturer, Salix Pharmaceuticals of Morrisville, N.C., to develop what's called a risk evaluation and mitigation strategy, to distribute a medication guide for patients so they're aware of the risk of kidney injury, and to conduct a post-marketing clinical trial to gauge the risk of injury.

The FDA also said it was concerned about similar non-prescription treatments, such as Fleet Phospho-soda, made by C.B. Fleet Co. of Lynchburg, Va. These treatments also shouldn't be used for bowel cleansing, and will get new warnings. At lower doses, however, they are safe for use as laxatives, the FDA said.

Patients routinely take oral sodium phosphate products -- either prescription or over the counter -- to clean the intestines before a colonoscopy and other medical procedures.

"The FDA is recommending that consumers not use the over-the-counter preparations for bowel cleansing, but that these products be used under the direction of a physician," Korvick said.

She added that there are alternatives to these preparations that can be used for bowel cleansing, including GoLYTELY and HalfLytely Bowel Prep.

In 2006, the U.S. Food and Drug Administration put out an alert on oral sodium phosphate products, excluding OsmoPrep, recommending that they be "used with caution" among patients with impaired kidney function due to their high phosphate content.

The FDA said Thursday that oral sodium phosphate products shouldn't be used by children under 18 years of age, or in combination with other laxatives containing sodium phosphate. And the agency urged the following high-risk groups to use the products with caution:

• people over 55 years of age,
• people who suffer from dehydration, kidney disease, acute colitis, or delayed bowel emptying, and,
• people taking certain medicines that affect kidney function, such as diuretics (fluid pills), angiotensin converting enzyme inhibitors (medications that lower blood pressure), angiotensin receptor blockers (used to treat high blood pressure, heart or kidney failure), and possibly nonsteroidal anti-inflammatory drugs (similar to ibuprofen and other arthritis medications).

Earlier this year, researchers reporting in the Archives of Internal Medicine said the risks of oral sodium phosphate solutions and some oral sodium phosphate tablets were rare but real, particularly for elderly patients.

Dr. Hemant K. Roy, an associate professor in the department of medicine at Evanston-Northwestern Healthcare in Illinois, writing in an accompanying editorial in the journal, described the findings as "quite alarming."

However, he stressed that warranted concerns about phosphate solutions shouldn't keep patients from undergoing colon cancer screening.

"Colonoscopies save lives," he said. "We know it works. So this should not dissuade people from doing one. I think we just need to be cautious about the type of preparation we use and who we give it to, so that an extraordinarily rare complication is avoided. And we have options, so there is a way to do that."

According to the American Cancer Society, colorectal cancer ranks third in the United States in terms of cancer diagnoses among both men and women. The organization estimates that about 150,000 people will develop the disease this year alone.

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