Pain relievers like Tylenol contain an ingredient that not only eases physical pain but also robs the user of positive feelings of joy.

The acetaminophen in the drug was shown to numb the strong emotions in users making them indifferent to pleasant and disturbing photos.

This is the first time the side-effect has been documented of a drug that has been in use for 70 years, showing its potential as an "emotion reliever".

Research had shown that acetaminophen also works on psychological pain but the present study focused on positive emotions, says Geoffrey Durso, lead author of the study and a doctoral student in social psychology at The Ohio State University.

"This means that using Tylenol or similar products might have broader consequences than previously thought," Durso said.

Durso conducted the study with Andrew Luttrell, another graduate student in psychology at Ohio State, and Baldwin Way, an assistant professor of psychology and the Ohio State Wexner Medical Center's Institute for Behavioral Medicine Research.

Their results appear online in the journal Psychological Science.

Way said people in the study who took the pain reliever didn't appear to know they were reacting differently. "Most people probably aren't aware of how their emotions may be impacted when they take acetaminophen," he says.

The study involved 82 participants, half of whom took an acute dose of 1000mg of acetaminophen and half who took an identical-looking placebo. They then waited 60 minutes for the drug to take effect.

They then viewed 40 photographs selected from a database (International Affective Picture System) used by researchers around the world to elicit emotional responses.

From extremely unpleasant snaps of crying, malnourished children to neutral ones like a cow in a field to the very pleasant, the pictures elicited lower ratings by the group who took acetaminophen.

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," Way says.

Acetaminophen may be blunting individuals' broader judgments of everything, not just emotional response, says Durso.

However, a more specific study asking the participants to evaluate emotions and how much blue they saw in each photo showed that judgments of blue colour content were similar in all.

This suggests that acetaminophen affects our emotional evaluations and not our magnitude judgments in general.

The researchers plan to check other pain relievers such as ibuprofen and aspirin for similar blunting of emotions.

Psychological theory believes that certain factors control how we react to the bad things that happen in life but this study suggests that common factors may influence how sensitive we are to both the bad as well as the good things in life.

Acetaminophen is the most common drug ingredient in the United States, found in more than 600 medicines. Each week about 23% of American adults (about 52 million people) use a medicine containing acetaminophen, the Consumer Healthcare Products Association reports.