By Alan Mozes
WEDNESDAY, April 15, 2015 (HealthDay News) -- Acetaminophen, the painkiller best known to Americans as Tylenol, may do more than simply dull pain -- it may also dull happy or sad emotions, new research finds.
The new, small study is the first to suggest that acetaminophen ratchets down a patient's emotional response to positive, upbeat stimulation. But the study builds on prior research into negative emotions, explained study lead author Geoffrey Durso.
"Recent research in psychology has found that acetaminophen blunts the extent to which individuals experience negative events beyond physical pain," said Durso, a doctoral student in social psychology at Ohio State University in Columbus. "Our study was inspired by asking why this might be the case."
The new study, published online recently in Psychological Science, involved two experiments, each enlisting about 80 college students.
In the first experiment, half of the participants took a 1000-milligram dose of acetaminophen, while the other half took a dummy pill. An hour later, all were shown 40 photographs designed to provoke emotional responses that ranged from positive (pictures of children playing with cute pets) to negative (photos of sickly, underfed children).
Participants ranked each photo's emotional content, and then indicated how each image made them feel.
The result: those who took acetaminophen offered more muted responses to both the negative and the positive images.
A follow-up study was structured exactly the same way, but also asked participants to indicate how much of the color blue they saw in each image. The goal was to see whether acetaminophen only affected emotions, or if it also affected the ability to cast accurate judgments overall.
The results found that acetaminophen had no impact on color assessment -- suggesting that only emotions were affected.
Overall, Durso said the study found a "reliable but relatively subtle" association between acetaminophen and a blunting of emotions. Just how the drug might do this remains elusive, however.
"Acetaminophen exerts a multitude of effects on the individual," Durso said. He believes the medicine could alter brain activity in various ways, such as tweaking activity of the neurochemical serotonin, reducing inflammatory signaling, or decreasing activation in areas responsible for emotion.
"Any one or combination of these effects could be responsible for the psychological outcomes that we observe on individuals' blunted negative and positive evaluations," he said.
But Durso stressed that acetaminophen's effect on emotions may not be unique -- other painkillers, such as aspirin or ibuprofen, might have similar effects, although that's not yet been tested.
McNeil Consumer Healthcare, the manufacturer of Tylenol, took issue with the findings. In a statement, the company said that the Ohio study has "a very small sample size, making it difficult to draw definitive conclusions about the impact of acetaminophen on the response to both positive and negative emotional stimuli."
Alan Hilfer is chief psychologist emeritus at Maimonides Medical Center in New York City. After reviewing the study, he called the findings both "interesting and surprising."
"I can't honestly think of another drug out there that has this type of side effect," he said. "At the same time, I'm struck by the fact that no one has ever reported feeling this type of emotional dulling after taking this medication. And [use of the drug is] very, very common," Hilfer added.
"I could speculate that because people sometimes do take acetaminophen to relax and sometimes to help them sleep, that perhaps there could be some impact over a short period of time during which there is a relaxing of the intensity of one's emotions as well," Hilfer said. "But that's strictly speculative on my part. And I would certainly want to see these findings replicated before suggesting that anyone alter their recommendations with respect to the use of over-the-counter Tylenol."