The objective of this study was to identify transformation products (TPs) of citalopram (CIT), an antidepressant drug, in laboratory experiments. Moreover, toxicity predictions and analyzes in wastewater samples were performed. For the formation of TPs, raw water was used for the processes of hydrolysis; photodegradation under ultraviolet (UV) irradiation and chlorination. The toxicities were predicted by computational toxicity assessment. The TPs were identified by liquid chromatography quadrupole time-of-flight mass spectrometry (LC-QTOF/MS) in broadband collision induced dissociation (bbCID) acquisition mode and product ion scan mode (MS/MS). The probable structures of the TPs under study were established based on accurate mass, fragmentations observed in the MS spectra and prediction tools software. The experiments resulted in seventeen possible identified TPs and their stability and formation was monitored over time in the experiments. Two of these TPs were identified in wastewater samples It was also observed that most of TPs formed were either less toxic then CIT or had a similar toxicity. Display omitted •Elucidated seventeen transformation products of citalopram by UHPLC-QTOF.•Hydrolysis, photodegradation and chlorination experiments under laboratory conditions.•Time profile of TPs formation were monitored.•In silico toxicity assessment resulted in TPs with mutagenic potentials.•Citalopram and two TPs were detected in wastewater samples.