A popular painkiller used by almost a quarter of all adults in the U.S. each week could have an unexpected side-effect by making people less capable of feeling empathy towards others. This finding, which is explained in a new study in the journal Social Cognitive and Affective Neuroscience, could have a range of potential consequences, particularly regarding social relationships and interactions.

Acetaminophen, also known as paracetamol, is found in more than 600 different medicines currently available in the U.S., which are used to treat everything from headaches to arthritis. The drug is one of the most effective painkillers on the market, although pain can sometimes be a good thing, as not only does it teach us to avoid danger, but it also enables us to understand what other people are feeling, thereby encouraging social harmony.

A number of previous studies have shown that, when we witness or imagine other people in pain, the patterns of activity that occur in the brain are very similar to those seen when we experience pain ourselves. This led the researchers behind this latest study to suspect that, when a person’s own receptiveness to pain is dulled, their ability to empathize may decrease as well.

To investigate, they recruited a number of volunteers to take part in a series of experiments, during which some were given a solution containing acetaminophen while others received a placebo. In the first of these tests, participants were asked to read hypothetical accounts of people in both physical and emotional pain after suffering either injuries or personal tragedies like the death of a close relative.

When asked to rate how much pain these fictitious characters were experiencing, those who had received acetaminophen gave much lower scores than those who had taken the placebo.

During the next experiment, participants were subjected to a loud noise blast, ranging from 75 to 105 decibels. Once again, those who had taken the painkiller rated these blasts as less painful than those in the placebo group, while also claiming that they would be less uncomfortable for another hypothetical person.

^{When empathizing with others, activity patterns in our brains are similar to when we experience pain ourselves. Federico Marsicano/Shutterstock}

Finally, the participants were placed into groups of four in order to play a game known as Cyberball, which involves throwing and catching a virtual ball between players. However, the program was rigged to ensure that one player never received the ball.

When asked to rate how much emotional pain this social ostracism would have caused the excluded player, those who had taken acetaminophen once more gave a lower score than those who had not.

Commenting on these results, study co-author Dominik Mischkowski stated that “other people's pain doesn't seem as big of a deal to you when you've taken acetaminophen.” The importance of this is summed up by Mischkowski’s colleague Baldwin Way, who explains that “if you are having an argument with your spouse and you just took acetaminophen, this research suggests you might be less understanding of what you did to hurt your spouse's feelings.”

Exactly how the drug blocks empathy is still somewhat unknown. Previous research has shown that the ability to feel for other people is at least partially regulated by a neurotransmitter called oxytocin, although as yet there is no evidence that acetaminophen affects this particular pathway.

Additionally, the study authors note that it is particularly interesting that acetaminophen affects our ability to empathize with both physical and emotional pain, and suggest that these two effects could be caused by separate mechanisms.