Powerful synthetic opiates are essential tools for capturing and handling wild animals. These drugs provide rapid and relatively long-lasting immobilization that can be completely reversed by specific opiate antagonists. Although some animals initially recover within minutes of receiving an antagonist, others will revert to a partially or completely narcotized state several hours later.¹

Factors believed to predispose or contribute to renarcotization include rapid metabolism of antagonists, deposition of opiates in subcutaneous tissues or fascial planes during injection, entero-hepatic shunting of opiates or metabolites, extremely high opiate doses and variable individual or species sensitivities to opiates.² Renarcotization can potentially compromise survival of affected individuals  and represents a shortcoming of opiate immobilization in wild animals. Using long-acting opiate antagonists should prevent or minimize renarcotization in wild animals immobilized with synthetic opiates.

Unfortunately, the effective durations of most available antagonists appear too short to completely prevent renarcotization in large wild animals immobilized with opiates. Renarcotization has been observed with the use of diprenorphine, nalmefene and ornaloxone HCL to antagonize opiates.³

Naltrexone HCL Sedation Reversal

Naltrexone HCL is an opioid receptor antagonist that is used in veterinary medicine to block receptors as a reversal agent for opiate agonists such as butorphanol. Used by zoos for sedation reversal in exotic animals, it is also used for the treatment of recurring, compulsive animal behavior disorders such as tail-chasing and self-mutilation such as acral lick dermatitis.⁴ Naltrexone is currently under study for possible application in a wide range of other therapeutic areas within veterinary medicine, such as in treating lymphoma, adenomas, nasal/sinus cancer, inflammatory bowel disease and degenerative myelopathy.

Pharmacology and Pharmacokinetics

​Naltrexone competitively binds to opiate receptors in the CNS, thereby preventing both endogenous opioids (eg, endorphins) and exogenously administered opioid agonists or agonist/antagonists from occupying the site. Naltrexone may be more effective in blocking the euphoric aspects of opioids and less effective at blocking the respiratory depressive or miotic effects.⁴

Naltrexone may also increase plasma concentrations of luteinizing hormone (LH), cortisol, and ACTH. In dogs with experimentally-induced hypovolemic shock, naltrexone (like naloxone) given IV in high doses increased mean arterial pressure, cardiac output, stroke volume, and left ventricular contractility.

Naltrexone is rapidly and nearly completely absorbed, but undergoes a significant first-pass effect as only 5% to 12% of a dose reaches the systemic circulation. Naltrexone circulates throughout the body and CSF levels are approximately 30% of those found in plasma. Only approximately 20% to 30% is bound to plasma proteins. It is unknown whether naltrexone crosses the placenta or enters milk. Naltrexone is metabolized in the liver primarily to 6-beta-naltrexol, which has some opioid blocking activity. The metabolites are eliminated primarily via the kidney. In humans, serum half-life of naltrexone is approximately 4 hours and approximately 13 hours for 6-beta-naltrexol.⁴

Effects of Naltrexone

The time from administration to naltrexone taking full effect is reported to be between 1 to 2 hours, after which clinical signs should improve. The effects of this medication are short-lived, meaning they will stop working within 24 hours, although the benefits may be prolonged if an animal has decreased kidney and/or liver function.⁴ Naltrexone may cause abdominal cramping, nausea & vomiting, nervousness, insomnia, joint or muscle pain, skin rashes, and itching. Liver toxicity is possible with high doses. Additional behavior modification techniques may be required to reduce undesired behaviors in small animals and exotics. 

Pregnant women and people who are allergic to this drug should be very careful not to accidentally take it. Because this drug has a higher risk for causing birth defects, pregnant women should wear disposable gloves when giving doses or handling the drug and avoid inhaling any dust from split or crushed tablets. Skin or eyes exposed to powder or liquids containing the drug should immediately be washed or rinsed off thoroughly.⁴

Where to buy Naltrexone

Naltrexone is available in the U.S. through pharmaceutical manufacturers and through veterinary custom compounding companies. NALTREXONE HCL 50 MG/ML by NexGen Pharmaceuticals provides excellent performance as an opioid receptor antagonist and animal anesthesia reversal, and in diagnosing and treating behavioral conditions in small and exotic animals.

Please consult your veterinarian prior to beginning any treatment regimen.

FOR RX ONLY: A valid prescription from a licensed veterinarian is required for dispensing this medication.

¹Haigh, J.C. 1982. Mammalian Immobilizing Drugs: Their Pharmacology and Effects. In: Chemical immobilization of North American Wildlife, Wisconsin Humane Society, Milwaukee, Wisconsin, pp.46-63.

²Miller, M. et. al. Efficacy And Safety Of Naltrexone Hydrochloride For Antagonizing Carfentanil Citrate Immobilization In Captive Rocky Mountain Elk (Cervuselaphusnelsoni). Journal of Wildlife Diseases, 32(2), 1996, 234-239.

³Jessup, D.A., et. al. 1985. Immobilization of free-ranging desert bighorn sheep, tule elk, and wild horses using carfentanil and xylazine reversal with naloxone, diprenorphine, and yohimbine. Journal of the American Veterinary Medical Association 187:1253-1254.

⁴Plumb’s Veterinary Drugs.