BACKGROUND Orthodontic patients often take analgesics for pain during treatment. But various analgesics have different capacities to inhibit prostaglandins, and these differences might affect tooth movement. The purposes of this study were to determine by direct measurement the effects that acetylsalicylic acid, ibuprofen, and acetaminophen have on orthodontic tooth movement in rats and to evaluate histologically the differences in bone resorption in the pressure area in rats treated with these analgesics. METHODS Thirty-six adult male Wistar rats were divided in 4 groups of 9 each. Orthodontic appliances were placed on the rats' incisors. In the 3 experimental groups, analgesics were diluted in reverse osmosis water and delivered via a gastric tube: 100 mg/kg acetylsalicylic acid, or 30 mg/kg ibuprofen, or 200 mg/kg acetaminophen. Control animals received only the reverse osmosis water. At the end of the experimental period, the rats were killed and histological examinations were performed. RESULTS Analysis of variance showed statistically significant differences between the control group, which was given reverse osmosis water, and the groups given aspirin and ibuprofen. There were also statistically significant differences between the acetaminophen group and the ibuprofen and aspirin groups, respectively. There was no significant difference between the acetaminophen group and the control group, or between the aspirin and ibuprofen groups. Tooth movement was similar in the groups. CONCLUSIONS The results indicate that nonsteroidal anti-inflammatory analgesics such as aspirin and ibuprofen diminish the number of osteoclasts, probably by inhibiting the secretion of prostaglandins, thereby reducing orthodontic tooth movement. Acetaminophen did not affect orthodontic tooth movement in rats, and it might be the analgesic of choice for treating pain associated with orthodontic treatment.