Objective of this study was aimed to compare the therapeutic effects of 8 mg dexamethasone intramuscular administered pre-operatively vs post-operatively after the surgical extraction of impacted mandibular third molars, in Al-Qurayyat, Saudi Arabia, population. One hundred and fifty patients who visited the Department of Oral and Maxillofacial Surgery of Gurayat specialized dental center, Al-Qurayyat, Saudi Arabia, were included in the study. The study was conducted from February 2018 to August 2018. 150 patients constituted the study and were randomly divided into two groups. Group A received 8 mg dexamethasone (intramuscular) 1hr pre-operatively. Group B received 8 mg dexamethasone (intramuscular) immediately after the surgery. According to the gender there were 81(54%) males and 69 (46%) females, in both the groups combined. Males to females’ ratio was 1.17: 1 in this study. Post-operative 1st and 3rd day follow-up showed that there was a significant difference between the two groups in terms of swelling and mouth opening (P=0.000). Overall, Group A showed much better results as far as the post-operative swelling and mouth opening was concerned. There was no statistical difference in the scores when the post-operative pain was evaluated on 1st, 3rd and 7th day (P=0.679, P=0.755, P=0.202). The depth, angulation and the position of the tooth did not affect the study in terms of significance (P=0.626, P=0.874, P=1.000). It was concluded that preoperative administration of single dose of dexamethasone intramuscularly was more effective than the postoperative period, in reducing the swelling and trismus.