Background: People with intellectual and developmental disabilities (IDD) may be at risk for medication-related problems due to complex medication regimens. The objective of this study was to determine the medication regimen complexity of adults with IDD. Methods: This retrospective study compared the Medication Regimen Complexity Index (MRCI) scores and number of medications of patients with IDD to a group without IDD (GenMed group), as well as a subanalysis between type of IDD. Results: The MRCI score of the IDD group (29.0 + 8.1, n = 200) was significantly higher than that of the GenMed group (16.7 ± 12.0, n = 200), p < 0.001. IDD patients had significantly more medications (11.2 ± 6.5) compared with the GenMed group (7.4 ± 5.1), p < 0.001. There were no significant differences between the number of medications prescribed or the MRCI scores between types of IDD. Conclusion: Clinicians must be aware that patients who have IDD have more complex medication regimens and higher potential for medication-related problems.