Acetaminophen is the most commonly used OTC medication in the country—it is an active ingredient in many brands of pain relievers, not just Tylenol—and acetaminophen overdose is one of the leading causes of acute liver failure. Many Americans may be unknowingly increasing their risk.

For the study, researchers at Northwestern University in Illinois interviewed 500 adult patients at general medical clinics in Atlanta and Chicago. The survey showed that more than half of them had used acetaminophen in the past six months, and 19 percent said they were heavy users or took acetaminophen every day or a couple of times per week.

Researchers then asked patients if they understood what the recommended dosage of products was and if they were able to take them safely.

Researchers found that nearly 25 percent of patients were at risk for overdosing by exceeding the recommending dose of 4 grams in one 24-hour period. Five percent of patients took more than 6 grams in the same time span. In addition, nearly half of the patients were using two products containing acetaminophen at the same time.

“Our findings suggest that many consumers do not recognize or differentiate the active ingredient in [over-the-counter] pain medicines, nor do they necessarily closely adhere to package or label instructions,” said Michael Wolf, an associate professor of medicine at Northwestern. “Given the prevalence of the problem, risk of significant adverse effects and lack of a learned intermediary, we believe this to be a serious public-health threat requiring urgent attention.”

Researchers warn that patients should treat OTC medications like any other prescribed medication and carefully read and adhere to instructions.