Researchers studying the commonly used pain reliever acetaminophen found it has a previously unknown side effect: It blunts positive emotions.
In the study, participants who took acetaminophen reported less strong emotions when they saw both very pleasant and very disturbing photos, when compared to those who took placebos.
Acetaminophen, the main ingredient in the over-the-counter pain reliever Tylenol, has been in use for more than 70 years in the United States, but this is the first time that this side effect has been documented.
Previous research has shown that acetaminophen not only reduces physical pain, but can also ease the psychological pain of rejection and even tamp down feelings of existential dread. This study takes those results one step further by showing that the pain reliever also reduces feelings of pleasure, said Geoffrey Durso, lead author of the study and a doctoral student in social psychology at The Ohio State University.
“This means that using Tylenol or similar products might have broader consequences than previously thought,” Durso said in a statement. “Rather than just being a pain reliever, acetaminophen can be seen as an all-purpose emotion reliever.”
For the study, 82 college students took either 1,000 mg of acetaminophen or an identical-looking placebo before viewing a series of photos. Some of the photos were pleasant (e.g. children playing with cats), while others were unpleasant (e.g. malnourished children) and some neutral (e.g. cow in a field).
The students rated each photo for how positive or negative it was on a scale of -5 (extremely negative) to 5 (extremely positive). The students also rated their emotional response to each photo on a scale of 0 (no emotional response) to 10 (extreme emotional response).
The results revealed that students who had taken acetaminophen rated both the unpleasant and pleasant photos less extremely than those who had taken the placebo. That is, they rated the negative photos as being less negative and the positive photos as being less positive.
Acetaminophen also seemed to lower the students’ emotional response to the photos. While those who had taken acetaminophen reported an average emotional response of 5.85, those who had taken the placebo reported a response of 6.76.
“People who took acetaminophen didn’t feel the same highs or lows as did the people who took placebos,” said co-author Dr. Baldwin Way, an assistant professor of psychology and a member of the Ohio State Wexner Medical Center’s Institute for Behavioral Medicine Research.
The researchers aren’t exactly sure what explains the strange effects, but they say several different mechanisms could be involved.
Acetaminophen exerts a multitude of effects, including changing serotonin neurotransmission in the brain, reducing inflammatory signaling in the brain, and decreasing activation in the brain areas responsible for emotion, for instance. Any one of these effects — or a combination of them — could be responsible for the observed blunting of negative and positive evaluations, the researchers said.
At this point, the researchers don’t know if other pain relievers such as ibuprofen and aspirin have the same effect, although they plan on studying that question, Durso said.
Acetaminophen, unlike many other pain relievers, is not a nonsteroidal anti-inflammatory drug, or NSAID. That means it not thought to control inflammation in the body. Whether that fact has any relevance to possible emotional effects of the drugs is still an open question, Durso said.
These results may also have an impact on psychological theory, said Dr. Way. An important question in psychological research is whether the same biochemical factors control how we react to both positive and negative events in our lives. A common theory is that certain factors control how we react to the bad things that happen in life — for example, how devastated people feel when they go through a divorce.
But this study offers support to a relatively new theory that says that common factors may influence how sensitive we are to both the bad as well as the good things in life. That means the person who is more devastated by a divorce may thrive more than others when they get a promotion at work or have some other extremely positive event happen.
In this study, acetaminophen may have tapped 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 Durso.
The new research was published online April 10 in the journal Psychological Science.