The aim of this study was to evaluate the anti-inflammatory effect of a single 4 mg injection of submucosal betamethasone after extraction of impacted third molars. Single-centre, single-blinded, parallel group study; Forty-three patients were submitted to impacted third molar extraction. In this study, 4 mg single-dose submucosal betamethasone was injected in the interventional group, while in the control group nothing was injected. Postoperative measurement included pain via the VAS scale, swelling and trismus with facial measurements and maximum mouth openings, and finally nerve sensitivity. There was a significant difference between the two groups regarding trismic pain and edema. The use of a single 4 mg submucosal betamethasone injection leads to a reduction of oedema, trismus and pain in patients undergoing impacted third molar extraction. .