WASHINGTON - US health officials said on Thursday they will add the sternest safety warnings available to prescription drugs used to cleanse the bowel before colonoscopies.

The federal Food and Drug Administration said it has received more than 20 reports of a rare but serious form of kidney failure among patients who have taken the drugs, known as oral phosphate products.

The new boxed warning label will apply to Visicol and OsmoPrep, both prescription tablets made by Salix Pharmaceuticals. The label warns that the drugs should be used with caution in older patients, those that suffer from dehydration and kidney disease or those that take medications that affect the kidneys.

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Regulators said they also worry about kidney risks from over-the-counter bowel cleansers, such as Fleet Phospho-soda, made by C.B. Fleet Co Inc.

While many phosphate products sold without a prescription are labeled only to treat constipation, regulators said they often are used at higher doses to clear the colon.

'The available data do not show a risk of acute kidney injury when these 'over-the-counter' products are used at the lower doses for laxative use,' the FDA said in a statement. 'However, when used for bowel cleansing, these products have the same risks as prescription' drugs.

The agency advised consumers not to use any of the over-the-counter products for bowel cleansing. While non-prescription products cannot receive boxed warnings, FDA officials said they plan to require stricter labeling on the dozens of over-the-counter phosphate drugs.

The FDA first warned doctors and patients about potential kidney risks with the medications in 2006. -- AP