Over the past decade, increasing off-label use of quetiapine has been reported worldwide from various sources. We wanted to investigate how this is reflected in therapeutic drug monitoring (TDM) data. Requisitions for serum concentration measurements of quetiapine from a TDM service in Central Norway during 2001-2019 were obtained, and analyzed for trends in quetiapine doses, serum concentrations, demographics and indicators of diagnoses. 19759 samples from 7459 individuals were included. Daily doses of quetiapine decreased by 24 mg per year (95% CI -25.61 - -21.48, p<0.001, N=4505). A corresponding decrease in quetiapine serum concentrations was not seen. The proportion of requisitions with diagnoses indicating reimbursable use, i.e. within approved indications, was 13 % for the whole study period. Mean daily doses were slightly higher in the reimbursable group, but declined over time in these samples, as well. To our understanding, these results provide additional documentation of increasing low-dose usage of quetiapine, and signal both increasing off-label use, as well as possibly higher dose-intake than prescribed and incorrect filing of reimbursable use in this population. There is a continued need for investigation and characterization of quetiapine use and prescription practices, and the extent of and motivations for off-label prescription of the drug.