Naloxone is an opiate antagonist which is used intravenously in emergency situations to reverse the respiratory depression caused by overdoses of heroin, morphine or other opioids. Naloxone has not been linked to serum enzyme elevations during therapy or to clinically apparent liver injury.


Naloxone (nal ox’ one) is a derivative of the natural plant alkaloid thebaine and is similar to oxymorphone. Naloxone, however, is a competitive antagonist of the opiate receptors and has particularly high affinity for the μ opiate receptor and can displace morphine and other full agonists, thereby reversing their effects. Naloxone causes rapid onset of withdrawal symptoms in opioid dependent persons and is generally used in emergency rooms to treat the respiratory depression caused by opioid overdose or postoperatively to reverse opioid anesthetic effects. Naloxone has sometimes been provided to emergency medical personnel and police and fire personnel for use onsite in emergency situations. Naloxone was first approved for use in the United States in 1971. It remains available as a solution for injection in generic forms and under the brand name Narcan, typically in concentrations of 0.4 mg/mL to be given intramuscularly or intravenously. It is also available as a nasal spray in solution of 4 mg/0.1mL. The usual initial dose in adults with suspected opioid overdose is 0.4 to 2.0 mg. Naloxone is also available in fixed combination with buprenorphine for sublingual use (Suboxone and generic) and with pentazocine for oral use. Naloxone is poorly absorbed by the sublingual or oral route and is added to discourage the intravenous or parenteral administration of buprenorphine or pentazocine. Side effects of naloxone in opioid dependent persons include mood changes, sweating, anxiety, restlessness, trembling, dizziness, flushing, headache, nausea, vomiting, cardiac tachyarrhythmias, seizures, chest pain and acute pulmonary edema—symptoms of acute opioid withdrawal. Naloxone has minimal effects in persons not taking opioids. Naloxone is not a controlled substance, but its use is sometimes restricted to medical staff trained in emergency medicine or anesthesia.


Therapy with naloxone has not been linked to serum enzyme elevations or to idiosyncratic acute, clinically apparent liver injury. Patients with opioid overdose often have underlying chronic liver diseases such as alcoholic liver disease, hepatitis B or C, but treatment with naloxone does not appear to exacerbate those conditions. Naloxone is extensively metabolized in the liver, but largely by conjugation with glucuronide followed by its urinary excretion.

Likelihood score: E (unlikely cause of clinically apparent liver injury).

Drug Class: Opioid Antagonists; see also Substance Abuse Treatment Agents

Other Drugs in the Class: Nalmefene; Naloxegol, Naltrexone



Naloxone – Generic, Narcan®


Opioid Antagonists


Product labeling at DailyMed, National Library of Medicine, NIH



References updated: 24 March 2020

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    (Expert review of hepatotoxicity published in 1999; mentions that trials of naltrexone have reported serum aminotransferase elevations in up to 30% of recipients, an effect that appeared to be partially dose dependent; naloxone not discussed).
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