IntroductionAttention deficit hyperactivity disorder (ADHD) affects 5%–10% of paediatric population and is reportedly more common in children with type 1 diabetes (T1D), exacerbating its clinical course. Proper treatment of ADHD in such patients may thus provide neurological and metabolic benefits. To test this, we designed a non-commercial second phase clinical trial comparing the impact of different pharmacological interventions for ADHD in children with T1D.Methods and analysisThis is a multicentre, randomised, open-label, cross-over clinical trial in children and adolescents with ADHD and T1D. The trial will be conducted in four reference paediatric diabetes centres in Poland. Over 36 months, eligible patients with both T1D and ADHD (aged 8–16.5 years, T1D duration >1 year) will be offered participation. Patients’ guardians will undergo online once-weekly training sessions behaviour management for 10 weeks. Afterward, children will be randomised to methylphenidate (long-release capsule, doses 18-36-54 mg) versus lisdexamphetamine (LDX, 30-50-70 mg). Pharmacotherapy will continue for 6 months before switching to alternative medication. Throughout the trial, the participants will be evaluated every 3 months by their diabetologist and online psychological assessments. The primary endpoint (ADHD symptom severity, Conners 3.0 questionnaire) will be assessed by a blinded investigator. Secondary endpoints will include HbA1c, continuous glucose monitoring indices and quality-of-life (PedsQL).Ethics and disseminationThe trial is approved by Bioethical Committee at Medical University of Lodz and Polish regulatory agency (RNN/142/22/KE, UR/DBL/D/263/2022). The results will be communicated to the research and clinical community, and Polish agencies responsible for healthcare policy. Patient organisations focused on paediatric T1D will be notified by a consortium member. We hope to use the trial’s results to promote collaboration between mental health professionals and diabetes teams, evaluate the economic feasibility of using LDX in patients with both diseases and the long run improve ADHD treatment in children with T1D.Trial registration numbersEU Clinical Trials Register (EU-CTR, 2022-001906-24) andNCT05957055.