OBJECTIVES: In January 2008, the Food and Drug Administration (FDA) communicated concerns and later in May 2009, issued a warning about an increased risk of suicidality related to all antiepileptic drugs (AEDs). The purpose of this study is to evaluate the association between an FDA suicidality warning and AED use among Oklahoma Medicaid enrollees diagnosed with epilepsy and/or psychiatric disorder(s). METHODS: A longitudinal interrupted design was conducted to study Oklahoma Medicaid claims data from January 2006 through December 2009. A total of 13,126 individuals met the study criteria: diagnosis with epilepsy and/or psychiatric disorder(s) and filling of at least one AED prescription. A segmented logistic regression model compared the level and trend in the log odds of AED use among three time periods: a baseline period of 25 months (Jan. 2006 - Jan. 2008) before the FDA warning; the 16 months (Feb. 2008-May 2009) during the FDA warning; and the 7 months (June 2009-Dec. 2009) after the FDA warning. Generalized estimation equations (GEE) were used to estimate trends in AED utilization while adjusting for several covariates. RESULTS: There was a statistical increase in the trend, expressed as a monthly change in log odds of AED use, before the FDA warning period (p<0.0001). However, this trend decreased by 34% (99% CI: 10.2% to 57.6%, p=0.0002) during the FDA warning period when compared to the baseline trend. This decrease in trend did not remain significant after the FDA warning period (p=0.2957). Compared with the baseline level of AED utilization before the FDA warning period, the log odds of AED utilization level also decreased by 22% (99% CI: 1.9%  42.2%, p=0.0048) after the FDA warning period. CONCLUSIONS: The FDA suicidality warning was associated with a reduction in overall AED use among this population.