Last July, pharmaceutical giant Johnson & Johnson began lowering recommended dosages on Tylenol labels when the U.S. Food and Drug Administration found that unintentional acetaminophen overdoses accounted for more than 40 percent of acute liver failure cases in a six-year time span. But a recent study suggests that people still don’t know how much is too much.

Northwestern University researchers asked 500 adult patients at outpatient general medicine clinics how often they’d used acetaminophen in the past six months. About 19 percent admitted to being “heavy users,” which means they took acetaminophen every day or a couple of times a week. Nearly 50 percent of the study participants said they’d taken two acetaminophen-containing products at the same time.

Considering the fact that one Extra Strength Tylenol capsule contains about 500 milligrams of acetaminophen, it was easy for a quarter of the study participants to nearly overdose by exceeding the recommended daily dose of 4,000 milligrams. Five percent of the participants took more than 6 grams (6,000 milligrams) in a 24-hour period.

“We believe this to be a serious public-health threat requiring urgent attention,” said associate professor of medicine Dr. Michael Wolf.