Researchers instructed 106 study participants to take 4 grams of Tylenol (eight extra-strength tablets) a day for two weeks, with some taking only Tylenol, and some taking Tylenol combined with an opioid painkiller. The rest of the participants were given a placebo.
Nearly 40 percent of the participants taking Tylenol or the Tylenol/opioid combination displayed abnormal liver test results that indicated liver damage, while those taking the placebo showed no damage.
The study's co-author, Dr. Neil Kaplowitz of the University of Southern California said, "I would urge the public not to exceed 4 grams a day. This is a drug that has a rather narrow safety window." Kaplowitz added that heavy drinkers should not exceed 2 grams a day.
Tylenol maker McNeil Consumer & Specialty Pharmaceuticals said it had conducted its own research that tracked high-dose Tylenol users over longer periods than the Kaplowitz study, and found that its product did not lead to liver disease.
Kaplowitz and co-author Dr. Paul Watkins of the University of North Carolina were hired by Purdue Pharma, which makes the prescription painkiller OxyContin, to see why people testing a drug containing acetaminophen and the opiate hydrocodone were having abnormal liver tests. Contrary to the researchers' theories, they found that acetaminophen was the culprit.
Acetaminophen is Americans' over-the-counter painkiller of choice. Acetaminophen overdose is also the leading cause of acute liver failure.