BACKGROUND Although the detrimental health effects of cigarettes are well known, many people, including some orthodontic patients, continue to smoke. Nicotine can affect bone resorption and apposition. Therefore, the aim of this study was to investigate the effect of nicotine on orthodontic tooth movement. METHODS Thirty-two adult male Sprague-Dawley rats were randomly divided into 3 experimental groups and 1 control group. With the rats under general anesthesia, 5-mm nickel-titanium closed-coil springs were placed between the maxillary right first molars and first incisors. Injections of nicotine were administered daily for 13 days: group A, 0.5 mg per kilogram; group B, 0.75 mg per kilogram; and group C, 1 mg per kilogram. Group D, the control group, received injections of 0.1 mL of normal saline solution. At the end of the experimental period, the rats were killed, and interproximal tooth movements were measured. RESULTS The mean amounts of tooth movement were 0.47 mm in group A, 0.62 mm in group B, 0.78 mm in group C, and 0.21 mm in group D. Statistical analysis with analyis of variance (ANOVA) test showed significant differences between all groups. The most movement occurred in group C, and group D had the least movement. CONCLUSIONS Nicotine accelerates orthodontic tooth movement, and this effect is dose-dependent. Nicotine's role in accelerating tooth movement might be related to its effect of accelerating bone resorption.