To describe the cerebral haemodynamic changes that occur in patients presenting to the emergency department (ED) after synthetic cannabinoid (SC) use in order to clarify the pathophysiology of neurologic adverse effects of SCs.Observational study.Emergency Department of Kocaeli Derince Training and Research Hospital, Turkey, from June 2017 to January 2018.Patients in whom tetrahydrocannabinol was detected in the urinalysis due to SC use were included in the study. Healthy controls were also included. All the participants were evaluated using transcranial Doppler ultrasonography (TCD). The peak systolic flow rate, mean flow rate, pulsatility index and resistivity index of the right and the left anterior carotid arteries, middle cerebral arteries and posterior cerebral arteries were measured.A total of 74 patients were included in this study - 38 patients in the SC group and 36 patients in the control group. The mean age of all of the subjects was 29.6 ±9.1 years, whereas, 91.9% of them were males. In the synthetic cannabinoids group, statistically significant decrease in the cerebral blood flow peak systolic and mean values as well as increases in the pulsatility index; and resistivity index were found via TCD when compared to the control group.Cerebral blood flow and resistance changes due to SC use are likely to play a role in the main pathogenesis of the neurological symptoms, increasing the frequency of ischemic or haemorrhagic strokes.