Purpose: The purpose of this study was to compare antipsychotic use by youth with intellectual/developmental disabilities to youth without IDD as they transitioned from pediatric to adult health care services. In addition, antipsychotic use was compared between youth with different types of IDD (autism, Fetal Alcohol Syndrome, Down syndrome and 'other'). Methods: Population level administrative health data was used to compare the dispensing of antipsychotics for youth aged 15 to 24 years with and without IDD, between 2010 and 2019. Because antipsychotics are associated with metabolic syndrome and type 2 diabetes, we also examined the dispensing data for metformin. In addition, we examined dispensed antipsychotics between different types of IDD. For both dispensing of antipsychotics and metformin, we used multi-variable logistic regression to derive adjusted odds ratios. Results: There were 20,591 youth with IDD and 1,293,791 youth without IDD. Youth with IDD had significantly higher odds of being dispensed an antipsychotic (7.13 (6.82, 7.44)), even when a diagnosis of a psychotic illness was included in the regression. Higher odds were found in all age groups. Youth with IDD also had significantly higher odds of being dispensed metformin (3.739 (3.323, 4.208)) compared to youth without IDD. Youth with autism, FAS and 'other' types of IDD diagnoses all had higher odds of being dispensed an antipsychotic compared to youth with Down syndrome. Conclusion: Compared to youth without IDD, youth with IDD were more frequently dispensed antipsychotics and metformin. These findings have important implications for the health of youth with IDD.