The Assumption Nobody Questions
Pain relievers relieve pain. You take two pills, the headache fades, and everything else about the way you think and feel stays the same. Acetaminophen, the active ingredient in Tylenol and more than 600 over-the-counter products, has occupied that position of comfortable familiarity since its FDA approval in 1955, taken weekly by roughly 52 million American adults.
What the label does not mention is that the drug appears to dampen something most people would never associate with a pill bottle: the ability to feel other people's pain.
The Painkiller-to-Empathy-Killer Pipeline
The reasoning starts with neuroimaging. Functional MRI studies had established that experiencing your own pain and empathizing with someone else's pain activate overlapping brain regions, principally the dorsal anterior cingulate cortex and the anterior insula (Eisenberger et al., 2003). Dominik Mischkowski and colleagues at Ohio State University hypothesized that if a drug suppresses processing in those circuits, it should not be selective about whose pain it dampens (DOI: 10.1093/scan/nsw057).
In Experiment 1, 80 university students took either 1,000 mg of acetaminophen or a visually identical placebo, then one hour later read scenarios describing someone experiencing physical pain (a knife cut) or social pain (a friend excluding them). Across both types, the acetaminophen group reported lower empathic concern, lower perceived pain on behalf of the person described, and less personal distress. Mood did not change; the drug was not making people happier or sadder, just making other people's suffering register less.
Experiment 2 raised the stakes with 114 participants and live stimuli: subjects watched a Cyberball game where one player was deliberately excluded, then imagined another study participant receiving painful noise blasts. Acetaminophen reduced empathic concern for both the ostracized player and the participant receiving the blasts, and also reduced the unpleasantness of noise blasts delivered to participants themselves. Mediation analysis showed that this self-focused pain reduction partially explained the empathy effect, suggesting the drug worked by making participants less reactive to painful stimuli across the board, whether the pain was their own or somebody else's.
Effect sizes were moderate for empathic affect (roughly Cohen's d ≈ 0.5) and small for empathic cognition (d ≈ 0.2), per the authors' meta-analysis across both experiments, and gender did not moderate the effect in either study.
It Extends Beyond Pain
Three years later, the same team found the problem ran deeper when a third double-blind RCT with 114 new participants showed that acetaminophen also reduced positive empathy: the warm vicarious pleasure you feel reading about someone else's good fortune (DOI: 10.3389/fpsyg.2019.00538). The drug did not just numb responses to suffering; it blunted the capacity to share in joy.
This aligned with Durso, Luttrell, and Way's 2015 finding that acetaminophen flattens emotional reactions to both negative and positive stimuli (DOI: 10.1177/0956797615570366), suggesting not a targeted analgesic with a clean mechanism but a broad emotional dampener sitting in nearly every medicine cabinet on the continent.
Converging evidence arrived from two directions. A 2023 EEG study found that 1,000 mg of acetaminophen altered mu rhythm suppression, a neural marker of sensorimotor resonance tied to empathy, while participants viewed images of people in pain (DOI: 10.1016/j.neuropsychologia.2023.108544). And in 2018, researchers at Dokuz Eylül University demonstrated dose-dependent reductions in empathy-like behavior in rats given acetaminophen, establishing that the effect crosses the species barrier (DOI: 10.1016/j.pbb.2018.10.004).
A Number Worth Pausing Over
Fifty-two million weekly users. A standard 1,000 mg dose lasts 4 to 6 hours. If one-seventh of those users take the drug on any given day, roughly 7.4 million Americans may be walking around at any given moment with a pharmacologically induced reduction in their capacity to respond to other people's distress. No other psychological side effect of any over-the-counter medication operates at comparable scale.
The Strongest Case Against These Findings
A 2026 PRISMA-guided systematic review published in Pain examined 50 studies on how pharmacological pain manipulations, including acetaminophen, opioids, cannabinoids, and placebo analgesia, affect social emotions and behavior. The conclusion was sobering: results across the field were "inconsistent, with findings generally showing small effects in both directions," only 24% of included studies confirmed that their manipulation actually reduced first-hand pain, and the strongest, most consistent empathy effect came not from acetaminophen but from placebo analgesia.
The review called for "well-powered studies, large-scale replications, and systematic meta-analyses," which is a polite acknowledgment that the existing evidence base, while suggestive, cannot support confident causal claims at the population level. All three core acetaminophen-empathy RCTs come from the same research group, and independent large-scale replications with pre-registered protocols remain absent.
What We Didn't Prove
The two 2016 experiments totaled 194 participants, all university students at a single institution, and the 2019 study added another 114 undergraduates from the same pool. These are WEIRD samples (Western, Educated, Industrialized, Rich, Democratic). Whether the effects generalize to older adults, clinical populations, or people taking acetaminophen for actual pain remains untested.
A Cohen's d of 0.5 means the acetaminophen and placebo groups overlapped substantially; this is not a binary on/off switch for empathy. The 2023 EEG study provides neural convergence and the rat study offers cross-species support, but neither was a pre-registered replication of the original human paradigm.
No study has examined whether repeated daily use produces cumulative empathy deficits or whether tolerance develops; the lab studies tested single doses in drug-naive participants. The gap between "a single dose reduces empathy in a lab for a few hours" and "millions of weekly users are walking around with diminished compassion" is enormous, and the research has not bridged it.
The Bottom Line
Acetaminophen works as a painkiller. Nobody disputes that. But three RCTs, an EEG study, and an animal model suggest it also dampens the ability to feel other people's pain and share in their pleasure, a side effect that never appeared on the label because nobody thought to look for it until 2010. The effect sizes are real but moderate, the replication base is young, and a 2026 systematic review found the broader field's results inconsistent. This is not settled science. It is science that should unsettle anyone who takes the drug without thinking about what else it might be doing.
What You Can Do
If you use acetaminophen regularly: You do not need to stop, since the empathy reduction is moderate, temporary, and tied to the drug's 4-to-6-hour active window. But be aware that your emotional responses to other people may be slightly blunted during that period, particularly your capacity to recognize and respond to others' distress.
If you're choosing between pain relievers: NSAIDs like ibuprofen and naproxen have not shown comparable effects on empathy in the published literature. If emotional responsiveness matters for the day's tasks (parenting, caregiving, therapy, conflict resolution), that is one factor among many to weigh alongside each drug's other risks and benefits.
If you prescribe or recommend acetaminophen: The FDA mandates labeling for hepatotoxicity risk, but no guidance addresses psychological side effects. Acetaminophen is routinely recommended as first-line therapy for populations where empathy is already under strain: chronic pain patients, post-surgical patients, elderly residents of care facilities. The gap between what the research suggests and what the label discloses has persisted for over a decade.