Use it correctly and acetaminophen, best known by the Tylenol brand, lives up to its reputation as one of the safest painkillers. It's taken by some 100 million people a year, and liver damage occurs in only a small fraction of users.
But it is damage that can kill or require a liver transplant, damage that frustrated liver specialists insist should be avoidable.
The problem comes when people don't follow dosing instructions, or unwittingly take too much, not realizing acetaminophen is in hundreds of products, from the over-the-counter remedies Theraflu and Excedrin to the prescription narcotics Vicodin and Percocet.
"The argument that it's the safest sort of has overruled the idea that people cannot take any amount they feel like," says Dr. William Lee of the University of Texas Southwestern Medical Center, who laments that acetaminophen is popped like M&Ms.
Acetaminophen bottles currently recommend that adults take no more than 4,000 milligrams a day, or eight extra-strength pills.
Just a doubling of the maximum daily dose can be enough to kill, warns Dr. Anne Larson of the University of Washington Medical Center.
Yet, "if two is good, 10 is better in some patients' minds," she says with a sigh.
The Food and Drug Administration has long wrestled with the liver risk, warning two years ago that more than 56,000 emergency-room visits a year are due to acetaminophen overdoses and that 100 people die annually from unintentionally taking too much.
A study published this month by Larson and Lee has agency officials weighing whether to revisit the issue.
Over six years, researchers tracked 662 consecutive patients in acute liver failure who were treated at 22 transplant centers. (Acute liver failure is the most severe type, developing over days, unlike chronic liver failure that can simmer for years because of alcohol abuse or viral hepatitis.)
Almost half were acetaminophen-related. More remarkable was the steady increase: Acetaminophen was to blame for 28 percent of the liver poisonings in 1998, but caused 51 percent of cases in 2003.
That makes acetaminophen the most common cause of acute liver failure, the researchers report in the journal Hepatology.