Sex-specific effects of intranasal oxytocin on thermal pain perception: A randomised, double-blind, placebo-controlled cross-over study

Chronic neck and shoulder pain (CNSP) is a common musculoskeletal disorder in adults, which is linked to hypersensitivity to noxious stimuli. The hormone oxytocin has been implicated as a potential therapeutic for the management of chronic pain disorders, and has been suggested to have sex-specific effects on the salience of threatening stimuli. This study investigated the influence of intranasal oxytocin on the perception of noxious thermal stimuli. Participants were 24 individuals with CNSP lasting >12 months (eight women), and 24 age- and sex-matched healthy, pain-free controls. In a randomised double-blind, placebo-controlled, cross-over study, participants attended two sessions, self-administering intranasal oxytocin (24 IU) in one session, and placebo in another. Participants rated intensity and unpleasantness of thermal heat stimuli at three body sites: the cervical spine, deltoid, and tibialis anterior, on 11-point numerical rating scales. Compared with placebo, intranasal oxytocin increased the perceived intensity of noxious heat stimuli in women with CNSP (Cohen’s d = 0.71), but not in men with CNSP, or healthy, pain-free controls. Men and women displayed divergent sensitivity across target sites for ratings of pain intensity (partial eta squared = 0.12) and pain unpleasantness (partial eta squared = 0.24), irrespective of drug condition. Men were more sensitive at the cervical spine and deltoid, whereas women were more sensitive at the tibialis. These findings suggest that oxytocin and endogenous sex hormones may interact to influence the salience of noxious stimuli. The hyperalgesic effects of oxytocin in women suggest that caution should be taken when considering oxytocin in the management of chronic pain.