We’ve likely all used over-the-counter (OTC) painkillers at some point in our lives — whether for headaches, toothaches, fevers, or swollen knees. There are two main groups of OTC painkillers: acetaminophen (Tylenol), and non-steroidal anti-inflammatory drugs (NSAIDs) like aspirin, naproxen, and ibuprofen.

Despite the fact that acetaminophen has been an accepted painkiller for over a century, researchers still don’t quite understand how it works. We know it numbs pain, but its pathways aren’t always clear. And lately, research has shown that the painkiller has an effect on other aspects of the brain aside from pure physical pain. For example, researchers found that acetaminophen dulls emotions in what they referred to as a “blanketing effect,” though they’re still not sure about the mechanisms behind it.

In a new study published in Social Cognitive and Affective Neuroscience, researchers have found yet another effect of acetaminophen on the mind — it impairs a person’s ability to detect errors. In the past, studies “have revealed that acetaminophen reduces the magnitude of reactivity to social rejection, frustration, [and] dissonance,” the study abstract said. “Given this diversity of consequences, it has been proposed that the psychological effects of acetaminophen may reflect a widespread blunting of evaluative processing.”

They chose to dig a little deeper into the brain mechanisms behind the effects of acetaminophen. “The core of our study is that we don’t fully understand how acetaminophen affects the brain,” said Dan Randles, a postdoctoral fellow at the University of British Columbia and an author of the study, in a press release. “While there’s been recent behavioral research on the effects of acetaminophen, we wanted to have a sense of what’s happening neurologically.”

For the study, the researchers gathered two groups of 30 participants each and had them complete a target-detection task. One group was given a normal dose of acetaminophen (1,000 milligrams) and the other was not. Participants in both groups were attached to electroencephalograms (EEG) in order to measure brain activity. The task involved hitting a "Go" button each time the letter "F" appeared on the screen, and then not hitting the button if an "E" flashed on the screen. Participants needed to act quickly to hit Go, or refrain when appropriate.

The researchers paid close attention to certain brain waves known as error-related negativity (ERN) and error-related positivity (Pe), which typically increases when someone realizes they’ve made a mistake. The participants given acetaminophen showed less Pe when they made mistakes compared to those who weren’t given the meds.

“It looks like acetaminophen makes it harder to recognize an error, which may have implications for cognitive control in daily life,” Randles said. “Sometimes you need to interrupt your normal processes or they’ll lead to a mistake, like when you’re talking to a friend while crossing the street, you should still be ready to react to an erratic driver.”

The researchers aren’t entirely sure how acetaminophen impairs error detection. It’s possible that the drug affects pathways that moderate both physical pain and social pain, as one 2009 study found. Back then, researchers discovered that physical pain and social rejection or simply hurt feelings shared neural processes, ones that can be impacted by taking Tylenol. Researchers have considered the mind effects of acetaminophen to be similar to that of Xanax or alcohol — having a dulling effect on both physical aches and mental anxiety.

It’s also possible that the meds can have an effect on concentration. In the most recent study, on top of having reduced Pe waves after taking acetaminophen, participants also tended to miss more "Go" buttons — hinting that the meds might also impair focus. But it's still not understood why this happens; the researchers plan on investigating this further in future studies.

“An obvious question is if people aren’t detecting these errors, are they also making errors more often when taking acetaminophen?” Randles asked. “This is the first study to address this question, so we need more work and ideally with tasks more closely related to normal daily behavior.”

Source: Randles D, Kam J, Heine S, Inzlicht M, Handy T. Acetaminophen attenuates error evaluation in cortex. Social Cognitive and Affective Neuroscience. 2016.