Possible side effects include cardiac effects such as bradycardia and QT interval prolongation. Neuroleptic malignant syndrome and QT interval prolongation may occur. The degree of QT prolongation determines the risk of syncope. ECG monitoring is recommended due to the possibility of QT interval prolongation. QT prolongation may be congenital or acquired. Serious side effects may include liver problems, QT prolongation, and seizures. This is due to prolongation of the QTc interval. The MRI may show swelling of the cerebral cortex, transient T2 prolongation, and transient lesions. Side effects may include sleepiness or QT prolongation. Serious side effects include QT prolongation and severe allergic reaction. Severe side effects may include urinary retention, QT prolongation, hallucinations, and glaucoma. Serious side effects may include low blood pressure, liver problems, psychosis, and QT prolongation. Diphenhydramine in overdose may occasionally result in QT prolongation. Serious side effects may include QT prolongation. Hypokalemia leads to characteristic ECG changes (QRS prolongation, ST-segment and T-wave depression, U-wave formation).