Acetaminophen potentiates, rather than diminishes, emotional responses to threatening stimuli

The most widely used over-the-counter painkiller in the United States, acetaminophen (paracetamol, Tylenol), has recently been shown to produce a variety of unintended psychological effects by attenuating neural activity in the cerebral cortex. As a result, prior research has suggested that acetaminophen blunts overall evaluative and emotional processing. In particular, one study found that acetaminophen diminished evaluation sensitivity to both positive and negative visual stimuli. Extending upon this work, the current study intersects with the fields of evolutionary psychology and psychopharmacology by investigating whether acetaminophen modulates responses to threatening stimuli that vary in ancestral and evolutionary relevance. Using a double-blind placebo-controlled study, we examined whether acetaminophen modulates the evaluation sensitivity to snakes and spiders (ancestral threats) and syringes and guns (modern threats). Based on previous research, we expected that participants in the acetaminophen condition would rate these threatening stimuli both less negatively and less emotionally arousing. Contrary to this hypothesis, we found that acetaminophen potentiated, rather than diminished, evaluations of threatening stimuli. Specifically, participants in the acetaminophen condition indicated that the threatening stimuli induced a greater emotional reaction. We also hypothesized that the prewired neural substrates governing responses to ancestrally relevant fear-related stimuli (e.g., snakes and spiders) would be less susceptible to the neuromodulation produced by this drug. Results showed that ancestral threats were rated as significantly more negative and emotionally arousing than modern threats. However, there was no interaction between the stimulus category and drug treatment, suggesting that acetaminophen influences perceptions of ancestral and modern threats in a similar manner. The findings indicate that acetaminophen does not have a general blunting effect when it comes to evaluative and emotional processing. In the case, acetaminophen appears to heighten our sensitivity to fear-inducing stimuli.