Naloxone
**Naloxone** is a pure opioid antagonist used primarily in veterinary medicine to reverse the effects of opioid agonists, specifically respiratory depression, sedation, and dysphoria. It is considered a life-saving antidote in cases of opioid overdose. Beyond its primary use as a reversal agent, naloxone is being investigated for the treatment of various forms of shock (septic, hypovolemic, or cardiogenic) and may be employed as a diagnostic or therapeutic tool to curb self-mutilating behaviors (like tail-chasing) by blocking endogenous endorphins. **Clinical Pearl:** Naloxone has no analgesic activity of its own. When used to reverse opioid-induced respiratory depression, it will simultaneously reverse the analgesic effects of the opioid, potentially leading to a sudden return of pain and subsequent catecholamine release.
Mechanism: Naloxone acts as a competitive antagonist at opioid receptors. It binds primarily to the **mu (ฮผ)** receptor, but also has affinity for **kappa (ฮบ)** and **sigma (ฯ)** receptors. By occupying these receptors without activating them, it displaces opioid agonists โ **rapid reversal of opioid-induced respiratory and CNS depression**. At very high doses, naloxone may exhibit additional pharmacologic activity, including **GABA antagonism** and the ability to increase dopamine levels.
Dosing by species
- Opioid reversal ยท 0.01-0.02 mg/kg ยท IM, IV, or SC ยท As needed
- Neonatal opioid reversal (following caesarian sections) ยท 1 drop of naloxone (0.4 mg/mL from a 1-mL syringe) ยท Sublingual ยท As needed ยท Administered under the tongue.
- Adjunctive treatment of hyperthermia ยท 0.01 mg/kg ยท IM or SC ยท Once ยท Used in cats with body temperatures exceeding 41.1ยฐC (106ยฐF) following anesthesia; temperature returned to normal in <30 minutes.
- Opioid overdose / reversal ยท 0.01-0.02 mg/kg ยท IV ยท prn ยท Short duration (30-40 min) ยท Repeated doses may be required for longer-acting opioids.
- Opioid overdose / reversal ยท 0.04 mg/kg ยท IM/SC/intratracheal ยท prn ยท Short duration
- Prolonged opioid antagonism ยท 0.02 mg/kg/h ยท IV ยท CRI ยท As needed ยท Administered as a continuous rate infusion if a longer duration of opioid antagonism is required.
- Opioid reversal ยท 0.01-0.022 mg/kg ยท IV ยท As needed ยท To reverse sedative and excitatory effects of narcotic agonists. Note: ARCI UCGFS Class 3 Drug.
- Limit increases in locomotor activity secondary to narcotic agonists ยท 0.01 mg/kg ยท IV ยท As needed
- Treatment of opioid gastrointestinal tract dysfunction ยท 10-50 micrograms/kg ยท IV ยท As needed ยท Will induce movement and passage of contents.
Routes of administration
Contraindications
- Hypersensitivity to naloxone
- Indiscriminate use in animals that have undergone major surgery or trauma (due to severe pain from analgesia reversal)
Adverse effects
- Reversal of analgesia (return of pain)
- Acute withdrawal syndrome in opioid-dependent patients
- CNS excitement (especially if used in meperidine overdose)
- Seizures (at very high doses, likely secondary to GABA antagonism)
- Acute severe discomfort or pain (due to sudden loss of analgesia)
- Antanalgesic effects in opioid-naรฏve subjects
- Transient elevation of unconsciousness (at low doses)
Drug interactions
- Opioid partial-agonists (butorphanol, pentazocine, nalbuphine) ยท Naloxone may antagonize the effects of these agents (respiratory depression, analgesia). It should not be relied upon to treat respiratory depression caused by buprenorphine.
- Clonidine ยท Naloxone may reduce the hypotensive and bradycardic effects of clonidine; potentially useful for clonidine overdoses.
- Yohimbine ยท Naloxone may increase the CNS effects of yohimbine (anxiety, tremors, nausea, palpitations) and increase plasma cortisol levels.
- Opioid Agonists (e.g., Morphine, Methadone, Fentanyl) ยท Naloxone reverses the analgesic, sedative, and respiratory depressant effects of opioid agonists. ยท major
- Buprenorphine ยท Buprenorphine binds tightly to opioid receptors; naloxone may not fully reverse its effects or may require much higher doses. ยท moderate
Monitoring
- Respiratory rate and depth
- CNS function (level of sedation/arousal)
- Pain associated with opiate reversal
- Respiratory rate and effort
- Level of consciousness / sedation score
- Pain score (monitor for acute pain upon reversal)
- Heart rate and blood pressure
Overdose
Naloxone is considered a very safe agent with a very wide margin of safety. However, massive overdoses have initiated seizures in a few patients, which is theorized to be secondary to GABA antagonism.
VetSheet drug reference is intended for licensed veterinary professionals as a clinical decision-support aid, not a substitute for professional judgement or the manufacturerโs current label.