By Pat Anson, Editor

Health experts have been warning for years about the risk of liver damage caused by taking too much acetaminophen.  Now a new study is out that found a previously unknown side effect of the drug: It also dulls emotions.

Acetaminophen -- also known as paracetamol – is the world’s most widely used over-the-counter pain reliever. It is the active ingredient in Tylenol, Excedrin, and hundreds of other pain medications.

Researchers at Ohio State University conducted two studies involving over 80 college students, half of whom took a large dose of 1000 milligrams of acetaminophen and half who took a placebo. They waited 60 minutes for the drug to take effect.

The students then viewed 40 photographs from a database used by researchers to elicit emotional responses. The photographs ranged from the extremely unpleasant (crying, malnourished children) to the neutral (a cow in a field) to the very pleasant (young children playing with cats).

After viewing each photo, participants were asked to rate how positive or negative the photo was on a scale of -5 (extremely negative) to +5 (extremely positive). Then they viewed the same photos again and were asked to rate how emotional they felt, ranging from 0 (little or no emotion) to 10 (extreme amount of emotion).

Results in both studies showed that participants who took acetaminophen rated all photos less extremely than did those who took the placebo. Positive photos were not seen as positively under the influence of acetaminophen and negative photos were not seen as negatively. The same was true of their emotional reactions.

“People who took acetaminophen didn’t feel the same highs or lows as did the people who took placebos,” said Baldwin Way, an assistant professor of psychology at the Ohio State Wexner Medical Center’s Institute for Behavioral Medicine Research.

For example, people who took the placebo rated their emotional response relatively high (average score of 6.76) when they saw jarring photos of the malnourished child or the children with kittens. But people taking acetaminophen didn’t feel as much in either direction, reporting an average emotion level of 5.85 when they saw the same photos.

Neutral photos were rated similarly by all participants, regardless of whether they took the drug or not.

“This means that using Tylenol or similar products might have broader consequences than previously thought,” said Geoffrey Durso, lead author of the study and a doctoral student in social psychology at The Ohio State University.

“Rather than just being a pain reliever, acetaminophen can be seen as an all-purpose emotion reliever.”

Previous research has shown that acetaminophen reduces not only on physical pain, but also psychological pain.

“Most people probably aren’t aware of how their emotions may be impacted when they take acetaminophen,” said Way.

The study is published online in the journal Psychological Science.

Over 50 million people in the U.S. use acetaminophen each week to treat pain and fever. The pain reliever has long been associated with liver injury and allergic reactions such as skin rash. In the U.S. over 50,000 emergency room visits each year are caused by acetaminophen, including 25,000 hospitalizations and 450 deaths.