Drugs used before colonoscopies may cause kidney damage, the Food and Drug Administration warned on Thursday. Therefore they must carry a stern warning on the packaging.

The drugs the agency refers to are Visicol and OsmoPrep, both prescription drugs produced by Salix Pharmaceuticals. They both increase risk of acute phosphate nephropathy, a type of kidney injury. The label warns that the drugs should be used with caution in patients older than 55, those who suffers from dehydration and kidney disease and those who take medications that affect the kidneys.

Federal officials said they are also concerned about the risks of over-the-counter bowel cleansers, such as Fllet Phospho-soda, made by CB Fleet Company Inc.

“There are many people who used these and use them fine. But there are people who are developing severe kidney injury, and it’s important we try and identify who is at risk,” said Charles Ganley, who heads FDA’s office of nonprescription products.

The FDA said it had received 20 reports of kidney damage in people who had used the products since 2006. Three of those cases were confirmed by biopsy. Some cases happened within hours of use, while others were reported days or weeks later.

One reason for the risk might be that some people don’t drink enough fluid when they use oral sodium phosphate products for bowel cleansing, despite the instructions on the products.

The FDA’s warning doesn’t apply to Golytely, Nulytely, Colyte, Trilyte and Halflytely, which are polyethylene glycol preparations that can be used for bowel cleansing.