In a case of suspected serotonin syndrome after citalopram administration, what medication should be given intravenously?

In the context of serotonin syndrome following the administration of citalopram, the administration of an intravenous benzodiazepine, such as diazepam, is considered appropriate due to its calming effects and ability to alleviate muscle rigidity, agitation, and autonomic instability that can be present in this condition. Benzodiazepines help in stabilizing the patient by providing sedation and reducing overall sympathetic output, making it a cornerstone in the management of serotonin syndrome.

Other medications listed may not effectively target the symptoms associated with serotonin syndrome. For instance, flumazenil could precipitate seizures and is contraindicated in cases of suspected poly-drug overdose or in patients with benzodiazepine dependence. Haloperidol is an antipsychotic that might not be justified as a primary treatment for serotonin syndrome. Propranolol, a beta-blocker, generally could help with autonomic symptoms but does not address the core issues and acute symptoms experienced in serotonin syndrome. Therefore, the use of diazepam is a well-supported choice in managing this acute state.