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Description: Panksepp’s Brain Opioid Theory of Attachment (1978, Biol. Psychiatry) proposed that that endogenous opioids promote the formation and maintenance of social bonds across species by mediating feelings of pleasure and security experienced in the presence of others. A literature synthesis from our group later suggested that opioids promote approach in safe contexts, whilst mainly providing relief when the animal is in distress (Løseth et al, 2014, Front Behav Neurosci). Here, we conduct a systematic review and meta-analysis of opioid modulation of social bonds in healthy humans. Some recent human studies have used opioid antagonists to probe mu-opioid dependent social processes. For instance, Inagaki et al (2015, SCAN) gave naltrexone to participants over four days and found a significant increase in diary ratings of feeling ‘disconnected from others’ compared to placebo. An acute naltrexone dose yielded a similar finding in Inagaki et al (2020, JEPG) but no effect in Tarr et al (2017, AHBP). Tchalova & MacDonald (2020, PNEC) reported that participants pretreated with naltrexone both disclosed less in a self-disclosure task, and also failed to display a task-related mood improvement evident in the placebo group. We consider the evidence of opioid modulation of social bonds in healthy humans in relation to findings from i) non-human animals; and ii) non-social reward in humans.