Acetaminophen hints at more of its unexpected psychological effects.
First discovered in 1877, acetaminophen is one of the most common drug ingredients in America. In 2008, 24.6 billion (resource no longer available at www.fda.gov) doses were sold.
An estimated 23 percent of Americans (52 million people) use medicines containing acetaminophen each week.
Despite the drug's long and popular service to humanity, science is still learning about its effects. These new insights are coming in from a range disciplines, not least psychology.
Over recent years, researchers delving into the psychological effects of acetaminophen have unearthed some tentative but fascinating insights.
For instance, a team from the University of Kentucky found that acetaminophen not only reduced physical pain, it also reduced the psychological pain caused by social rejection.
Another study, carried out at The Ohio State University and published in June 2015, found that acetaminophen reduced people's evaluation and response to both negative and positive stimuli.
The current study, by the same Ohio team, follows on from these findings. This time, the emotion under scrutiny is empathy.
Acetaminophen and empathy
The researchers carried out a three-pronged investigation. The first session involved 80 participants. Half received a liquid containing 1,000 milligrams of acetaminophen; the other half, the control group, drank a placebo solution without any drug.
One hour later, each participant was given a series of short scenarios to read. The stories included characters who experienced some sort of pain - for instance, a serious knife wound or losing a loved one. The participants then rated the physical and emotional pain experienced by the characters.
The team found that the individuals who had consumed acetaminophen rated the pain of the characters in the story as less severe.
The second leg of the experiment involved 114 participants. As before, half were given acetaminophen and the other half, placebo. They were subjected to short, loud blasts of white noise. Next, they were asked to rate the unpleasantness of the noise and how unpleasant they thought the noise would be for an anonymous participant.
Those who had taken the drug rated the noise as less unpleasant, and they also believed it would be less unpleasant for the anonymous participant.
In the third, and final, part of the study, the participants were allowed to meet each other and mingle before being taken off to sit alone and watch - but not take part in - an online "game."
The participants were told that three of the people they had just met were playing. During the game, two of the participants excluded the third (although the participants were not really involved).
The individuals who were watching the game were asked to rate the emotional pain and hurt of the participant who was excluded. According to the senior author, Baldwin Way, "[...] those who took acetaminophen showed a reduction in empathy. They weren't as concerned about the rejected person's hurt feelings."
Why would acetaminophen reduce empathy?
At this stage, it is not clear why this reduction in empathy occurs. However, previous studies involving brain scans might give us a clue.
Researchers found that, when an individual feels pain, and when they imagine pain in someone else, it causes a response in similar parts of the brain. In other words, the regions of the brain involved in experiencing pain are also involved in imagining the pain of others.
Theoretically, one can imagine that if the same brain areas are used to both experience and imagine pain, a drug reducing real pain might also reduce imagined pain.
If the results are replicated, they would be highly relevant to the real world. After all, it would mean that one quarter of the population would be regularly taking a drug that reduces empathy.
"We don't know why acetaminophen is having these effects, but it is concerning."
Empathy is vital in everyday situations. If an individual is arguing with a loved one and has a reduced ability to see the other person's side of the story, resolution is, perhaps, less likely. We rely on empathy for social cohesion, at least in part.
Because the research was carried out on a relatively small group, larger studies will be necessary to draw solid conclusions, but it certainly makes for interesting reading.
The fact that we are still learning about this drug seems surprising. However, the mechanisms by which acetaminophen reduces pain and fever are still not precisely known, so there may well be more surprises further down the road.