Chicago ? Healthy adults taking maximum doses of Tylenol for two weeks had abnormal liver test results in a small study, researchers found, raising concerns that even recommended amounts of the popular painkiller might lead to liver damage.

In the study, 106 participants took 4 grams of Tylenol – equivalent to eight extra-strength Tylenol tablets – each day for two weeks. Some took Tylenol alone, and some took it with an opioid painkiller. Dummy pills were given to 39 others.

There were no alarming liver test results among the people who took the placebos. But nearly 40 percent of people in all the other groups had abnormal test results that would signal liver damage, according to the study that appears in today’s Journal of the American Medical Assn.

“I would urge the public not to exceed 4 grams a day. This is a drug that has a rather narrow safety window,” said a study co-author, Dr. Neil Kaplowitz of the University of Southern California. Heavy drinkers should take no more than 2 grams daily, he said.