Objective To compare the efficacy of amoxicillin pre and post to prevent postsurgical removal of third molars infection. Material and method The clinical trial was conducted in the Department of Dentistry Hospital I El Porvenir-ESSALUD (Trujillo, Peru), between July and November of 2013. The 99 patients were randomized into 3 groups: group 1 received 2 g of amoxicillin one hour before surgery, group 2 received 500 mg of amoxicillin and 500 mg immediately after and every 08 hours for 4 days, and group 3 was not given any antibiotic. The surgical procedure was standardized and performed by the same operator, who was blinded to the procedure. The efficacy in preventing infection was measured by three parameters: pain, oral temperature and discharge. To analyze the proportions comparison test was used with p < 0,005. Results No difference was found between amoxicillin before and after surgery to prevent post extraction of third molars infection (p = 0,60). Furthermore, no difference between those groups and the non-antibiotic administration group was found (p = 0,35 and p = 1,00; respectively). Conclusion The pre and postoperative administration of amoxicillin are not effective to prevent post extraction of third molars infection