Background: Methylphenidate is the most frequently prescribed medication for the treatment of ADHD in children and adolescents in many countries. Although many randomised controlled trials support short-term efficacy, tolerability, and safety, data on long-term safety and tolerability are scarce. The aim of this study was to investigate the safety of methylphenidate over a 2-year period in relation to growth and development, psychiatric health, neurological health, and cardiovascular function in children and adolescents., Methods: We conducted a naturalistic, longitudinal, controlled study as part of the ADDUCE research programme in 27 European child and adolescent mental health centres in the UK, Germany, Switzerland, Italy, and Hungary. Participants aged 6-17 years were recruited into three cohorts: medication-naive ADHD patients who intended to start methylphenidate treatment (methylphenidate group), medication-naive ADHD patients who did not intend to start any ADHD medication (no-methylphenidate group), and a control group without ADHD. Children with ADHD diagnosed by a qualified clinician according to the DSM-IV criteria and, in the control group, children who scored less than 1·5 on average on the Swanson, Nolan, and Pelham IV rating scale for ADHD items, and whose hyperactivity score on the parent-rated Strengths and Difficulties Questionnaire was within the normal range (<6) were eligible for inclusion. Participants were excluded if they had previously taken any ADHD medications but remained eligible if they had previously taken or were currently taking other psychotropic drugs. The primary outcome was height velocity (height velocity SD score; estimated from at least two consecutive height measurements, and normalised with reference to the mean and SD of a population of the same age and sex)., Findings: Between Feb 01, 2012, and Jan 31, 2016, 1410 participants were enrolled (756 in methylphenidate group, 391 in no-methylphenidate group, and 263 in control group). 1070 (76·3%) participants were male, 332 (23·7%) were female, and for eight gender was unknown. The average age for the cohort was 9·28 years (SD 2·78; IQR 7-11). 1312 (93·0%) of 1410 participants were White. The methylphenidate and no-methylphenidate groups differed in ADHD symptom severity and other characteristics. After controlling for the effects of these variables using propensity scores, there was little evidence of an effect on growth (24 months height velocity SD score difference -0·07 (95% CI -0·18 to 0·04; p=0·20) or increased risk of psychiatric or neurological adverse events in the methylphenidate group compared with the no-methylphenidate group. Pulse rate and systolic and diastolic blood pressure were higher in the methylphenidate group compared with the no-methylphenidate group after 24 months of treatment. No serious adverse events were reported during the study., Interpretation: Our results suggest that long-term treatment with methylphenidate for 2 years is safe. There was no evidence to support the hypothesis that methylphenidate treatment leads to reductions in growth. Methylphenidate-related pulse and blood pressure changes, although relatively small, require regular monitoring., Funding: EU Seventh Framework Programme., Competing Interests: Declaration of interests KKCM reports grants from the CW Maplethorpe Fellowship, the UK National Institute for Health and Care Research (NIHR), the EU Horizon 2020 Framework, and the Hong Kong Research Grant Council, and personal fees from IQVIA Holdings, outside the submitted work. AH has received compensation for serving as consultant or speaker for Shire–Takeda and Medice, unrelated to this work. TB served in an advisory or consultancy role for Eyelevel, Infectopharm, Lundbeck, Medice, Neurim Pharmaceuticals, Oberberg, Roche, and Takeda; received conference support or speaker's fee from Jansen, Medice, and Takeda; and royalities from Hogrefe, Kohlhammer, CIP Medien, and Oxford University Press; outside the submitted work. JB has been in the past 3 years a consultant, member of advisory board, or speaker for Takeda–Shire, Roche, Medice, Angelini, Janssen, and Servier. SC reports collaboration on projects from the EU Seventh Framework Programme and on clinical trials sponsored by Shire Pharmaceutical Company, Lundbeck, Otsuka, Janssen-Cilag, and Angelini. MD has received research funding from Takeda–Shire, outside the submitted work. RWD has received compensation for serving as consultant or speaker, or he or the institution he works for have received research support or royalties from: EU Seventh Framework Programme, US National Institute of Mental Health (NIMH), German Federal Ministry of Health–Regulatory Agency, German Federal Ministry of Education and Research, German Research Foundation, Volkswagen Foundation; Boehringer Ingelheim, Ferring, Janssen-Cilag, Lilly, Lundbeck, Otsuka, Servier, Shire, Sunovion–Takeda, and Theravance. He was a former employee in clinical CNS research of Eli Lilly until Aug 2008, and owns Eli Lilly stock (small part of the respective annual salary). BF has been a consultant or speaker for Abbvie, Actelion, Allergan, Almirall, Alnylam, Amgen, Astellas, Astrazeneca, Bayer, Biogen, Biopecs, Bioproject, Biotronik, BMS, Boehringer, Celgène, Daiichi-Sankyio, Ethypharm, Forestlab, Genevrier, Genzyme, Gilead, Grünenthal, GSK, Idorsia, IMS, Indivior, IQVIA, JNJ, Léo, Lilly, Lundbeck, Menarini, MSD, Novartis, Novonordisk, Otsuka, Pfizer, Pierre-Frabre, Recordati, Roche, SANOFI, Servier, Takeda, UCB, ViiV, and Wellmera. CH reports research funding from the NIHR including the Health Technology Assessment SATURN trial (grant ref: NIHR128472) comparing methylpheidate with guanfacine for children and young people with ADHD and tics. CH was chair of the NICE Guideline (CG155) for psychosis and schizophrenia in children and young people; member of the NICE ADHD Guideline Update committee (NG87) and is a member of Eunethydis and the Europhean ADHD Guideline Group. SM reports speaker's fee, travel support, and research support from Shire, outside the submitted work. AN reports research funding from the EU, the German Ministry of Health, and the German Federal Joint Committee, outside the submitted work. PN has been a consultant or speaker for Medice, Servier, and Egis Pharmaceuticals, outside the submitted work. ER received speaker's fee and travel support from Shire, outside the submitted work. ESB has received in the last 3 years speakers fees from Takeda and Medice and research support from QBTech. AZ served in an advisory or consultancy role for Angelini, EduPharma, Servier; received conference support or speaker's fee from Angelini and Janssen; participated in clinical trials conducted by Angelini, Janssen, Lundbeck, Otsuka, Roche, Sevier, and Shire; and received royalties from Giunti OS and Oxford University Press. ICKW reports research and educational funding from Amgen, Bristol Myers Squibb, Pfizer, Janssen, Bayer, GSK, Novartis, Takeda, the Hong Kong Research Grants Council, the Hong Kong Health and Medical Research Fund, the Hong Kong Innovation and Technology Commission, the NIHR, the EU, and the Australian National Health and Medical Research Council, and the expert testimony payment from the Hong Kong Court of Final Appeal; outside the submitted work. DC reports, in the past 3 years, a consultant, member of advisory board, or speaker role for Takeda–Shire, Medice, Novartis, and Servier. He has received royalties from Oxford University Press and Cambridge University Press; research support from the Australian National Health and Medical Research Council and the Royal Children's Hospital Foundation; and funding for the current study from the European Commission. All other authors declare no competing interests., (Copyright © 2023 Elsevier Ltd. All rights reserved.)