Aim: To compare the frequency and factors associated with diabetes medication-taking (depression, perceived side effects, self-efficacy and social support) in people with mild to moderate intellectual disability and those without intellectual disability. Methods: In stage 1 of this study, we collated information on diabetes medication-taking and associated factors in 111 people with diabetes: 33 adults with mild to moderate intellectual disability and 78 adults without intellectual disability. Validated instruments measuring medicine-taking, self efficacy, depressive symptoms, perceived level of social support and perceived side effects were administered in both groups. In stage 2, we used an abductive qualitative approach to triangulate stage 1 findings with carers responses (n = 12). Results: The instruments showed good internal reliability (Cronbach’s α = 0.7–0.9). Comparisons between people with intellectual disabilities and those without revealed similar frequency of medication-taking (70% vs 62%; P = 0.41). People with intellectual disabilities and diabetes had significantly higher depressive symptoms, as measured by the Glasgow Depression Scale for people with a Learning Disability (P = 0.04), higher levels of perceived side effects (P = 0.01), and lower confidence levels, as measured by the Perceived Confidence Scale (P = 0.01). The results of stage 2 showed how carers of people with intellectual disabilities and diabetes optimized medication-taking yet infrequently discussed the side effects of medicines. Conclusions: Further investigation of medication-taking and side effects may result in the development of an evidence-informed intervention to improve medicines safety in people with intellectual disabilities.