AbstractA correct use of inhaler devices is essential in chronic obstructive pulmonary disease (COPD) treatment. Critical errors were studied by analysing 659 video-recorded demonstrations of inhaler technique from 364 COPD patients using six different inhaler device models. The majority of the included patients used two (55%) or more (20%) device models. Overall, 66% of the patients made ≥1 critical error with at least one device model. The corresponding numbers for patients using 1, 2 and ≥3 device models were 43%, 70% and 86%, respectively. The only factor associated with making ≥1 critical error was simultaneous use of two (adjusted odds ratios (aOR) 3.17, 95% confidence interval (95% CI) 1.81, 5.64) or three or more (aOR 8.97, 95% CI 3.93, 22.1) device models. In conclusion, the proportion of patients making critical errors in inhaler technique was substantial, particularly in those using several different device models. To obtain optimal COPD treatment, it is important to assess a patient’s inhaler technique and to minimise the number of inhaler device models.