 Neurosurgical ICU patients often receive hyperosmotic diuretics or cerebrospinal fluid CSF drainage to decrease their intracranial pressure. It is also common to keep them under normovolemia or mild hypervolemia status for stable cerebral perfusion pressure CPP. These patients have large urine output on average, and can decrease the reabsorption of specific medicines by renal tubules. Besides, hypervolemia status might increase the volume of distribution for hydrophilic medicines, and lower their serum concentration. Therefore, the pharmacokinetic characteristics in this population may be different from general population, which impacts the efficacy and toxicity of medicines with narrow therapeutic index. 