Empathy is a fundamental social-cognitive ability, allowing one to infer about the affective state of another individual and resonate with it, which is crucial for interpersonal relationships. On a neural level, this ability to empathize with others might stem from so-called ‘shared representations’, patterns of neural activation recruited both during the firsthand, as well as the empathic experience of a specific affective state. Correlational evidence for this account using fMRI repeatedly showed overlaps in brain activity for firsthand and witnessed pain. However, despite this correlational support, there's a shortage of studies offering causal evidence. Some research inducing placebo analgesia has shown consistent reduction in behavioral ratings and neural activity for both firsthand and empathic pain experiences. Although their findings support the ‘shared representations’ theory, their use of placebo analgesia makes it difficult to infer about the origin of their effects – whether they are due do pharmacological mechanisms or other top-down cognitive processes. To bridge this gap, we conducted a double-blind, within-subjects study administering both an opioid antagonist (naltrexone) or an inactive placebo before an empathy for pain paradigm on two subsequent testing sessions. We collected data of 35 participants (21 female individuals) and found that naltrexone reduced both firsthand pain and empathy for pain. While contrary to our hypothesized direction, the directional coherence of this substance effect suggests a shared opioidergic mechanism between the firsthand experience of pain and empathy for pain.