The previously unknown side-effect means that over-the-counter painkillers are leaving users not only pain-free, but also emotionally numb.
In a study carried out by US researchers, volunteers who took paracetamol reported weaker feelings when they saw both pleasant and harrowing photographs.
“This means that using paracetamol might have broader consequences than previously thought,” said lead author Geoffrey Durso, a doctoral student in social psychology at The Ohio State University.
“Rather than just being a pain reliever, paracetamol can be seen as an all-purpose emotion reliever.”
Previous research had shown that the pain-killer works not only on physical pain, but also on psychological pain.
However the new study takes those results one step further by showing that it also reduces how much users actually feel positive emotions.
Dr Baldwin Way, assistant professor of psychology at Ohio State added: “Most people probably aren’t aware of how their emotions may be impacted when they take paracetamol.
“People who took acetaminophen didn’t feel the same highs or lows as did the people who took placebos.”
Researchers asked 82 volunteers to take 1000mg of paracetamol - the equivalent of two normal sized tablets - or a placebo.
After waiting 60 minutes for the drug to take effect, they were asked to rate 40 photographs on whether the image made them feel positive or negative.
The photographs ranged from the extremely unpleasant, such as crying, malnourished children, to neutral images, like a cow in a field and very pleasant, such as children playing with kittens.
Results showed that participants who took acetaminophen rated all the photographs less extremely than did those who took the placebo.
In other words, 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.
Those who took the placebo rated their level of emotion relatively high, with an average score of 6.76, when they saw the most emotionally jarring photos of the malnourished child or the children with kittens.
People taking acetaminophen didn’t feel as much in either direction, reporting an average level of emotion of 5.85 when they saw the extreme photos.
Neutral photos were rated similarly by all participants, regardless of whether they took the drug or not.
A second study included a question about how blue the sky was in a number of images to rate whether paracetamol was just blunting all judgements in general. However the researchers found there was no relationship suggesting the change was limited to emotions.
The team now plans to study whether pain relievers like ibuprofen and aspirin have the same impact. Previous studies have suggested that ibuprofen could be used to treat depression but researchers believed that was due to its anti-inflammatory properties. However paracetamol is not an anti-inflammatory.
The researchers believe that paracetamol may tap 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,” added Mr Durso.
The research was published in the online journal Psychological Science.
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