Facial expression in animals is a suggested method to assess the subtle facial changes associated with pain. This study aimed to evaluate postoperative pain and establish its correlation with facial action units (AUs) in female dogs undergoing elective ovariohysterectomy (OVH) treated with cannabidiol alone or in combination with meloxicam. Sixty healthy female dogs of different breeds, were randomly assigned into four groups according to the treatment. The control group (G₀: n = 15) received a saline solution as placebo; the meloxicam group (G_Mlx: n = 15) received meloxicam at a dose of 0.2 mg kg^-1 IV, followed by 0.1 mg kg^-1 every 24 h, for 48 h during the postoperative period; the cannabidiol group (G_CBD: n = 15) received 2 mg kg^-1 PO every 12 h; and the combination group (G_Mlx/CBD: n = 15) received both treatments.

The treatments were administered 30 min before starting the surgery. High-resolution videos were recorded for one minute at baseline and multiple time points up to 48 h post-surgery to quantify facial action units: AU101, AU143 + 145, AU12, ear position, facial muscle tension, orbital tightening, and lip tension. Data was analyzed with a mixed linear model. The frequency of AU143 + 145, AU12, ear position, tension of the facial muscles, orbital tightening, and tension of the lips significantly increased at 30 min, 1 h, 2 h, and 3 h post-surgery (p = 0.0001) in the control group.

Thus, facial expressions associated with acute pain in dogs were characterized by AUs such as blink (AU143 + 145), lip corner puller (AU12), ear position, head position, tension of the facial muscles, orbital tightening, and tension of the lips, which maintained a positive moderate correlation with UMPS scores, this association indicates that these facial changes are signs of acute pain in dogs.