What antidote is used for benzodiazepine overdose?

The antidote used for benzodiazepine overdose is Flumazenil. Flumazenil is a specific benzodiazepine receptor antagonist that works by competitively inhibiting the action of benzodiazepines at the central nervous system receptor sites. This reversal agent can rapidly counteract the sedative effects of benzodiazepines, thus restoring consciousness and respiratory function in patients who have overdosed.

It's important to note that Flumazenil is not without risks, as its use is generally contraindicated in patients with certain conditions such as mixed drug overdoses, especially if the benzodiazepine is being used to counteract seizure activity, or in patients with a known history of benzodiazepine dependence, as it can precipitate withdrawal seizures.

The other choices do not serve as antidotes for benzodiazepine overdose. N-acetylcysteine is the antidote for acetaminophen (paracetamol) overdose, sodium nitrate is used in cases of cyanide poisoning, and calcium channel blockers are medications that may be used to manage cardiac conditions but do not specifically counteract benzodiazepine effects. Hence, Flumazenil is the proper choice for treating benzodiazepine overdose.