Naloxone (Narcan) is a rapid-acting opioid antagonist that blocks the effects of opioids like heroin at opioid receptors. Naloxone is generally administered intramuscularly or intranasally in overdose situations. Fomepizole is used to counteract ethylene glycol (antifreeze) and methanol poisoning. Flumazenil is a benzodiazepine receptor antagonist used to treat benzodiazepine overdose. Physostigmine can be used to treat severe anticholinergic drug toxicity. Examples of anticholinergic drugs are diphenhydramine, amitriptyline, and atropine.