Background People with major neurocognitive disorder might be susceptible to drug-induced QT interval prolongation due to the presence of a number of concomitant risk factors. Objective The aim of this study was to investigate the prevalence of QT-prolonging drugs and QT-prolonging drug-drug interactions and associated factors among older people with major neurocognitive disorder. Methods In this register-based study, we obtained information regarding QT-prolonging drug use in a large population of older people with major neurocognitive disorder, through record linkage between the Swedish registry for cognitive/dementia disorders, and the Swedish Prescribed Drug Register. QT-prolonging drugs were identified according to the CredibleMeds online database and interactions using the Janusmed interaction database. Drug use was defined as one or more filled prescriptions during a 6-month timeframe, July 01 to December 31, 2017. Associations between people with a QT-prolonging drug and the factors of age and gender were analysed through multiple logistic regression. Results Of 35,212 people included in the study, 41.6% had one or more QT-prolonging drug prescribed. The most commonly prescribed drug was donepezil, with a prevalence of 25.0%, followed by citalopram and escitalopram, representing 14.5% and 3.9% of prescriptions in the study population, respectively. Significant associations were found between QT-prolonging drug use and the factors of younger age and female gender. The most prevalent interaction was between citalopram and donepezil (2.7%), followed by the combination of escitalopram and donepezil (0.7%). Conclusions In this population of older people with major neurocognitive disorder, QT-prolonging drugs and interactions that increase the risk of torsade de pointes were prevalent. Due to the presence of many risk factors in this population, it is important to continuously evaluate current QT-prolonging drugs and concomitant drug treatment in each individual.