IntroductionAnti-tumour necrosis factor (TNF) therapy has greatly improved treatment outcomes in patients with inflammatory bowel disease (IBD), but long-term use is associated with cutaneous reactions, susceptibility to infections and frequent injections or hospital visits. Several non-controlled studies have demonstrated that dose reduction is feasible for a subset of patients, provided that early detection of a disease flare is possible. Here, we aim to compare the effectiveness of interval lengthening with standard dosing in maintaining remission in young patients with IBD.Methods and analysisIn this international, prospective, non-inferiority, partially randomised patient preference trial, we aim to recruit 148 patients aged 12–25 years with luminal Crohn’s disease or ulcerative colitis in sustained remission (ie, three consecutive in-range faecal calprotectin (FC) results or recently confirmed endoscopic remission). In the interventional arm, the dosing interval will be lengthened from 8 to 12 weeks for infliximab users and from 2 to 3 weeks for adalimumab users. In the control group, standard dosing will be continued. Rapid tests will be performed for FC every 4 weeks and for anti-TNF trough levels every 12 weeks. The primary outcome is the cumulative incidence of out-of-range FC results at 48-week follow-up. Secondary endpoints include time to get out-of-range FC results, cumulative incidence of adverse effects, proportion of patients progressing to loss of response and identification of predictors of successful interval lengthening.Ethics and disseminationThe protocol has been approved by the Medical Ethics Review Committee of the University Medical Centre Groningen and is pending at the other participating centres. Results will be disseminated in peer-reviewed journals and presented at scientific meetings.Trial registration numberEudraCT number: 2020-001811-26; ClinicalTrials.gov Identifier: NCT04646187. Protocol version 4, date 17 September 2021.