UPDATE 1-FDA seeks less acetaminophen in prescription drugs

* FDA wants 325 mg limit

* Would affect combination painkillers like Percocet

* Action would help prevent liver damage-FDA

* OTC acetaminophen not affected by action (Adds FDA and company comments, background, byline)

By Susan Heavey

WASHINGTON, Jan 13 (Reuters) - U.S. health regulators want to limit the amount of acetaminophen in prescription pain medicines in an effort 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 (JNJ.N) Tylenol.

"Patients taking these prescription products often do not know they are taking acetaminophen at all," said Sandra Kweder, a deputy director at FDA's Center for Drug Evaluation and Research. "They don't realize that they're overdosing."

The move affects products made by Johnson & Johnson, Abbott Laboratories (ABT.N), Endo Pharmaceuticals Holding Inc (ENDP.O), Watson Pharmaceuticals Incorporated (WPI.N) 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, one of the most commonly used U.S. drugs, has long been known to cause liver damage if patients overdose.Such overdoses cause nearly 460 deaths a year plus tens of thousands of emergency room visits and hospitalizations, FDA said.

The ingredient is often combined with other medications such as cold medications for over-the-counter products or, for prescription versions, various levels of opioid painkillers.

About 200 million prescriptions a year are written for such combination versions, the FDA said.

The dose reduction is not expected to affect patients' pain relief, said FDA's Kweder, who added that "the amount of acetaminophen has gradually crept up over the years."

Nonprescription acetaminophen products already carry a warning about the risk of liver damage, which can lead to organ transplants or even death.

They are not affected by Thursday's request. Drugmakers selling over-the-counter versions can still market "maximum strength" 500 milligram versions of the drug, though companies can voluntarily stop doing so, Kweder said.

She added that unlike OTC acetaminophen products, prescription ones are not always clearly labeled as containing the ingredient, making them more susceptible to overdoses.

Critics have long pointed to the dangerous potential for patients to take a strong, over-the-counter version on top of cold medications that also contain the ingredient.

Dr. Sidney Wolfe, head of Public Citizen's Health Research Group, said while the prescription limits were a good step, the FDA exercised "inexcusably poor judgment" by not tackling the concern surrounding OTC acetaminophen medicines.

About 80 percent of the acetaminophen market is nonprescription products, while about 20 percent includes prescription combination products, according to the FDA.

The FDA began reexamining the dangers of acetaminophen 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 on Thursday. It had called for a ban of combination acetaminophen products, an action that FDA's Kweder said could have compromised patient access to pain relief.

Representatives for J&J and Abbott said they were evaluating the FDA's request. Endo spokesman Kevin Wiggins said it already offers a Percocet version with less acetaminophen and would work with the FDA to meet the 3-year deadline. (Reporting by Susan Heavey; Editing by Tim Dobbyn and Carol Bishopric)