Background:
Alcohol in sub-toxic dosages has appeared to slightly reduce experimental pain in psychophysical paradigms. However, this effect may also reflect impaired scaling performance in subjective ratings. To address this, we additionally assessed facial responses as a more direct and cognitively unbiased pain measure, while acknowledging the potential confound of alcohol's effects on motor inhibitory function.

Methods:
We investigated 41 healthy participants (22 females) in a randomised, double-blind, and placebo-controlled design; targeting two moderate breath-alcohol levels (0.6‰, 0.8‰). Before and after an alcoholic or placebo drink, painful heat stimuli were applied to the forearm. Facial responses were analysed using the Facial Action Coding System (FACS). Subjective responses were assessed using a Numerical Rating Scale (NRS). To control for alcohol's effects on motor inhibitory function, participants completed the antisaccade task, which assesses inhibitory control over reflexive motor responses (eye movement).

Results:
While pain ratings were unaffected, alcohol significantly affected facial responses to pain, with the high alcohol dose leading to increased facial responses. Moreover, alcohol also led to a decrease in inhibitory control, with poorer performance in the antisaccade task. Not surprisingly, we found a significant association between the alcohol-induced increase in facial responses and the alcohol-induced decrease in inhibitory control.

Discussion:
Alcohol-induced motor disinhibition likely enhanced facial responses to pain without altering the subjective pain experience. In consequence, individuals under the influence of alcohol may facially display stronger pain levels (than experienced), which should not be interpreted as intentional exaggeration by clinicians involved in pain assessment.

Significance Statement:
Subtoxic doses of alcohol are known to produce weak analgesic effects. In contrast, the facial responses to pain were elevated under alcohol in the present study; probably due to an alcohol-induced motor disinhibition. Thus, individuals under the influence of alcohol may be analgized while in parallel being facially overly pain responsive.