Introduction: Dexamethasone (DEX) is one of the most potent agents with demonstrated improved overall survival rates in leaukemia patients, but its use has been reported with increased cognitive and executive functional impairment. Despite its longstanding use, little is known about the mechanism by which it exerts its effects on the brain, this being hindered by its common use in typical multi-agent treatment regimens that administer a cocktail of drugs making it difficult to assess the individual effects of DEX on the brain. Indeed, the dose dependent negative side effects of DEX and corticosteroids in particular have been well demonstrated previously [1]. The effect of DEX on memory has indeed been demonstrated earlier using fMRI where altered glucocorticoids altered hippocampal and prefrontal activation and delayed recall [2,3]. We have applied a network connectivity approach to longitudinally assess how DEX affects the strength of the communication between different regions of the brain and hence lead to its deteriorating cognitive fingerprint. Methods: In this pilot healthy control study, twenty healthy adults (24 ± 0.6 years) were administered 12mg/day of DEX oraly for 5 days. Baseline neuropsychiatric assessments including the BPRS, DASS and the HADS were administered along with MRI scans, all of which were performed on Day 1 of the study, prior to starting DEX and then subsequently at Day 5 and Day 16 (which was considered the washout period of DEX relevant to the half life of the drug) (refer to Fig. 1 for study design). Throughout the whole study, monitoring of sleep efficiency was carried out with a Fitbit tracker. Structural, spectroscopic, and functional MRI data were obtained at each of the three time points. A structural sagittal T1- weighted MPRAGE sequence was acquired with the following parameters: 256mm FOV, TE/TR/TI = 1.9/2300/900 ms, 192 contiguous slices, with a resolution of 1 x 1 x 1 mm3. Functional connectivity resting state data was acquired with a single-shot gradient echo EPI sequence with TR=3000 ms, TE=30ms, voxel size =3.4mm isotropic, and number of measurements = 89. Brain volume and thickness estimates were obtained with Freesurfer, and images were processed with the longitudinal stream with an un-biased within-subject template space. Functional connectivity data was processed with a combination of FSL, ANTs and network measures were obtained with CPAC. Data from different timepoints were grouped to represent averaged connectivity maps. Results: Fig. 2 represents the changes in connectivity seen at Day 5 following DEX administration and Day 16 compared to Day 1 prior to DEX. A significant reduction in the number of connections is delineated between the thalamus, hippocampus, and cerebellum at Day 5, while an increase in connectivity was seen in the insula. The number of connections has been quantified and represented in Fig. 3. A reversibility of the diminished connectivity is demonstrated at Day 16 especially in the hippocampus. The reductions in connectivity in the above-mentioned areas is accompanied with a significant reduction in thalamic, hippocampal, and cerebellar volumetric measures. Volumetric reductions were however significantly recovered at Day 16. Discussion: The effect of glucocorticoids on brain function has been well established, especially their negative effect on executive and cognitive function. Our findings of reduced connectivity in multiple regions including the thalamus and hippocampus is consistent with previous findings. The mechanism of action may be linked to the density of glucocorticoid receptors in these regions. The uniqueness of this work lies in defining a specific pathway linking the cerebellum to the thalamus and cortical areas that are affected by DEX. Interestingly the reduction in connectivity is associated with a decrease in regional volumes that is specific to the thalamus, hippocampus, and cerebellum but not changes in other volumetric brain regions. Interestingly, the washout scan did show some recovery but not to a full extent with the connectivity which may suggest an incomplete washout of DEX but this is yet to be validated. The data presented does however support a physiological basis for the effect of DEX which can be detected by resting state connectivity.