U.S. health regulators are requesting a limit on the amount of acetaminophen in prescription pain medicines to curb the risk of liver damage.

The move announced on Thursday aims to limit combination drugs such as the opioids Percocet and Vicodin to 325 milligrams of acetaminophen per pill and calls for them to carry a “black box” warning about potential liver failure.

Some of these medicines now contain as much as 750 milligrams of acetaminophen, a drug also sold over the counter in lower doses as a generic painkiller and Johnson & Johnson’s Tylenol.

“This change will provide an increased margin of safety to help prevent liver damage due to acetaminophen overdosing, a serious public health problem,” the Food and Drug Administration wrote in a public notice.

The move affects products made by Johnson & Johnson, Endo Pharmaceuticals Holding Inc., Watson Pharmaceuticals Inc. and Boehringer Ingelheim’s Roxane Laboratories unit, among others.

FDA said it was asking affected companies to comply but stopped short of ordering the actions. Changes would be phased in over three years.

Acetaminophen has long been known to cause liver damage if patients overdose or take multiple products that contain it.

FDA began re-examining the issue in 2002 but at the time its outside advisers cited a lack of data to target dosing. Another advisory panel in 2009, however, recommended some of the changes requested Thursday.

Acetaminophen is one of the most commonly used U.S. drugs, according to the FDA, and is often combined with other medications. Prescription versions often include it with opioid painkillers, it added.

While about 80 percent of the acetaminophen market is nonprescription products, about 20 percent includes prescription combination products, according to the agency.