Paracetamol may blunt feelings of pleasure as well as pain

"Paracetamol may dull emotions as well as physical pain, new study shows'," The Guardian reports.

The story comes from research testing whether over-the-counter painkiller paracetamol can blunt not just the feeling of pain, but also emotions.

Half the study’s 80 participants were given a normal dose of paracetamol, while the other half took a placebo pill. They were then asked to view photos commonly used by researchers to test both positive and negative emotional responses. These included, for instance, unpleasant pictures of crying, malnourished children and pleasant images, such as children playing with cats.

The study found that those who had taken paracetamol reported slightly less intense reactions to the photos than those who had taken a placebo pill. They also found the photos less emotionally arousing.

The researchers speculated that paracetamol may affect signalling pathways inside the brain, which may have an effect on mood.

However, far more research is needed before any conclusions can be drawn as to whether the painkiller can dull emotional reactions, particularly real life events.

If you are taking paracetamol on a long-term basis due to a chronic pain condition, and feel that you are less emotionally engaged than you used to be, you could discuss alternative treatment options with your GP.

Where did the story come from?

The study was carried out by researchers from Ohio State University and was funded by the National Science Foundation Graduate Research Fellowship and the National Center for Advancing Translational Sciences.

The study was published in the peer-reviewed journal Psychological Science.

Most newspapers reported the research accurately, if uncritically, although The Guardian mentioned at the end of its story that the differences between the two groups were not large.

Confusingly for UK readers, the Mail Online’s report used the US generic name for paracetamol, which is acetaminophen, and also the US brand Tylenol. Although these were the names used in the US paper, it is usual practice to use UK generic names when reporting research for a UK audience.

What kind of research was this?

This was a randomised controlled trial (RCT) to test whether taking paracetamol can blunt emotional reactions to negative and positive images.

The authors say the drug has recently been shown to blunt people’s reactions to a range of emotionally negative stimuli, in addition to reducing physical pain. For example, they say it has been found to blunt feelings of hurt in social relationships and reduce the discomfort felt in making difficult decisions. They suggest that this may be due to its neurochemical effects on the brain, and the drug may reduce positive reactions as well as negative ones.

What did the research involve?

The researchers carried out two studies involving 82 college students in the first and 85 in the second. The students were randomly assigned to be given either 1,000 milligrams of paracetamol (the maximum dose) or an identical-looking placebo, both in liquid form. They then waited 60 minutes for the drug to take effect.

Participants then viewed 40 photographs selected from a database (International Affective Picture System) used by researchers to elicit emotional responses. These consisted of 10 extremely unpleasant photos (such as crying, malnourished children), five moderately unpleasant, 10 "neutral" images (such as a cow in a field), five moderately pleasant images and 10 extremely pleasant images (for example, young children playing with cats).

In the first study, 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). They were then asked to view all 40 images again in a different, random order and asked to rate how much the photo made them feel an emotional reaction, from 0 (little or no emotion) to 10 (an extreme amount of emotion).

In the second study, participants saw all the images again in a different randomised order and were asked to make the same judgments of evaluation and emotional reactions as in the previous study. Additionally, participants in this second study also reported how much blue they saw in each photo, using an 11-point scale from 0 (the picture has no blue colour) to 10 (the picture is 100% blue). This was to test whether paracetamol blunts individuals' broader judgments "of magnitude", not just of emotional content.

The researchers then calculated average scores for participants’ evaluations and emotional arousal toward all 40 pictures, and for evaluation and emotional arousal towards neutral, positive and negative images.

What were the basic results?

Results in both studies showed that, overall, participants who took paracetamol rated all the photographs less intensely than those in the placebo group.

In other words, they evaluated unpleasant stimuli (anything that triggers a psychological or physical response) less negatively, and pleasant stimuli less positively than those who took a placebo.

They also rated both positive and negative images as less emotionally arousing than those taking a placebo.

There was no difference between the two groups in the rating of the degree of colour in each image.

How did the researchers interpret the results?

The researchers conclude that paracetamol reduces the intensity of both negative and positive emotions. "Rather than being labelled as merely a pain reliever, acetaminophen [paracetamol] might be better described as an all-purpose emotion reliever," they argue.

They speculate that the drug elicits neurochemical changes which affect evaluative psychological processes and that it might change sensitivity to emotional stimuli more generally – for example, causing someone to feel less joy at a wedding.

Conclusion

This small study found some slight differences in the way a group of people taking paracetamol reacted to a range of images, compared to a group of people taking a placebo.

Though an RCT is the "gold standard" of studies for determining whether a medication causes an effect, both groups need to be evenly matched for a variety of potentially confounding factors.

There is no information about the participants, other than the impression that they are all students, as they were given course credits for partaking in the study. It is not clear whether the groups were matched in terms of age, sex, ethnicity, whether they had children, or indeed whether they liked cats. 

The study is interesting, but no conclusions can be drawn as to whether paracetamol can dull emotional reactions to real life events.

While learning more about different chemicals’ effects on the brain and mood could lead to new treatments, this research has no immediately obvious clinical implications.

Therefore, further research is required into the potential side effects of this popular and effective painkiller.