In a double-blind pharmacological study, we investigated whether modulation of the endogenous opioid system influences both first-hand pain and empathy for pain. The study was motivated by the shared representations account of empathy, which proposes partially overlapping neural and bodily representations responsible in experiencing pain oneself and empathizing with the pain of others.

To test this assumption, participants were invited to two lab sessions. In one session they received the opioid antagonist naltrexone, which blocks endogenous opioid receptors involved in pain modulation, while in the other session they received a placebo. We hypothesized that opioid blockade would increase first-hand pain sensitivity and, in line with shared representations accounts, also affect empathic responses to others' pain in a similar manner.

Contrary to expectations, naltrexone did not increase self-reported pain intensity. This finding, while surprising at first, still fits with the overall literature indicating that the effects of opioid antagonism via naltrexone (and naloxone as well) on pain in healthy individuals are highly variable and appear to depend on some contextual factors.

More importantly, however, the effects of naltrexone on first-hand pain did transfer to empathy. This effect was specific to the affective dimension of empathy, operationalized as the unpleasantness experienced when observing another person in pain. Cognitive-evaluative judgments of others' pain were not significantly affected.

Overall, our findings suggest a selective involvement of the endogenous opioid system in the affective-motivational component of pain empathy. They further support the notion that empathic responses rely on partially shared representations between self and other, while indicating that these shared mechanisms may be quite specific.

You can find the paper here