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Acetaminophen is the main ingredient in the over-the-counter pain reliever Tylenol

Painkillers not only dull physical pain, but they can also dull your emotions.

This is according to a new study that claims acetaminophen - the main ingredient in the over-the-counter pain reliever Tylenol - has the ability to weaken feelings of happiness and sadness.

Acetaminophen has been in use for more than 70 years, but this is the first time that this side effect has been discovered.

Previous research had shown that acetaminophen works not only on physical pain, but also on psychological pain.

This study takes those results one step further by showing that it also reduces how much users actually feel positive emotions, said Geoffrey Durso, lead author at the Ohio State University.

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

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

Baldwin Way, an assistant professor of psychology at Ohio State University 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 said.

There were two studies of college students. The first involved 82 participants, half of whom took an acute dose of 1000 milligrams of acetaminophen and half who took an identical-looking placebo.

They then waited 60 minutes for the drug to take effect.

Participants then viewed 40 photographs selected from a database used by researchers around the world to elicit emotional responses.

The photographs ranged from the extremely unpleasant, to the neutral and the very pleasant.

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).

Results showed that participants who took acetaminophen rated all the photographs less extremely than did those who took the placebo. The researchers don't know if other pain relievers such as ibuprofen and aspirin have the same effect, although they plan on finding out in future research

They then viewed the same photos again and were asked to rate how much the photo made them feel an emotional reaction.

Results showed that participants who took acetaminophen rated all the photographs less extremely than did those who took the placebo.

'People who took acetaminophen didn't feel the same highs or lows as did the people who took placebos,' Way said.

One possibility is that acetaminophen changes how people judge magnitude.

Scientists claim acetaminophen may blunt individuals' broader judgments of everything, not just things having emotional content.

The researchers did a second study in which they had 85 people view the same photos and make the same judgments of evaluation and emotional reactions as in the prior study.

Participants in this second study also reported how much blue they saw in each photo.

Once again, individuals who took acetaminophen had emotional reactions to both negative and positive photographs that were significantly blunted.

But judgments of blue colour content were similar regardless of whether the participants took acetaminophen or not.

At this point, the researchers don't know if other pain relievers such as ibuprofen and aspirin have the same effect, although they plan on studying that question.

Acetaminophen, unlike many other pain relievers, is not a nonsteroidal anti-inflammatory drug, or NSAID.

That means it not thought to control inflammation in the body. Whether that fact has any relevance to possible emotional effects of the drugs is still an open question, the researchers added.

In this study, acetaminophen may have tapped into the sensitivity that makes some people react differently to both positive and negative life events.

'There is accumulating evidence that some people are more sensitive to big life events of all kinds, rather than just vulnerable to bad events,' Durso said.