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naltrexone treatment 

Naltrexone Description

Naltrexone Hydrochloride Tablets USP, an opioid antagonist, are a synthetic congener of oxymorphone with no opioid agonist properties. Naltrexone differs in structure from oxymorphone in that the methyl group on the nitrogen atom is replaced by a cyclopropylmethyl group. Naltrexone Hydrochloride Tablets USP are also related to the potent opioid antagonist, naloxone, or n-allylnoroxymorphone. The chemical name for Naltrexone hydrochloride is Morphinan-6-one, 17-(cyclopropylmethyl)-4,5-epoxy-3,14-dihydroxy-, hydrochloride, (5α)-. 

Pharmacodynamic Actions

Naltrexone hydrochloride is a pure opioid antagonist. It markedly attenuates or completely blocks, reversibly, the subjective effects of intravenously administered opioids.

When coadministered with morphine, on a chronic basis, Naltrexone hydrochloride blocks the physical dependence to morphine, heroin and other opioids.

Naltrexone hydrochloride has few, if any, intrinsic actions besides its opioid blocking properties. However, it does produce some pupillary constriction, by an unknown mechanism.

The administration of Naltrexone hydrochloride is not associated with the development of tolerance or dependence. In subjects physically dependent on opioids, Naltrexone hydrochloride will precipitate withdrawal symptomatology.

Clinical studies indicate that 50 mg of Naltrexone hydrochloride will block the pharmacologic effects of 25 mg of intravenously administered heroin for periods as long as 24 hours. Other data suggest that doubling the dose of Naltrexone hydrochloride provides blockade for 48 hours, and tripling the dose of Naltrexone hydrochloride provides blockade for about 72 hours. 

Naltrexone hydrochloride blocks the effects of opioids by competitive binding (i.e., analogous to competitive inhibition of enzymes) at opioid receptors. This makes the blockade produced potentially surmountable, but overcoming full Naltrexone blockade by administration of very high doses of opiates has resulted in excessive symptoms of histamine release in experimental subjects.

The mechanism of action of Naltrexone hydrochloride in alcoholism is not understood; however, involvement of the endogenous opioid system is suggested by preclinical data. Naltrexone hydrochloride, an opioid receptor antagonist, competitively binds to such receptors and may block the effects of endogenous opioids. Opioid antagonists have been shown to reduce alcohol consumption by animals, and Naltrexone hydrochloride has been shown to reduce alcohol consumption in clinical studies.

Naltrexone hydrochloride is not aversive therapy and does not cause a disulfiram-like reaction either as a result of opiate use or ethanol ingestion.


Naltrexone hydrochloride is a pure opioid receptor antagonist. Although well absorbed orally, Naltrexone is subject to significant first pass metabolism with oral bioavailability estimates ranging from 5 to 40%. The activity of Naltrexone is believed to be due to both parent and the 6-ß-naltrexol metabolite. Both parent drug and metabolites are excreted primarily by the kidney (53% to 79% of the dose), however, urinary excretion of unchanged Naltrexone accounts for less than 2% of an oral dose and fecal excretion is a minor elimination pathway. The mean elimination half-life (T-1/2) values for Naltrexone and 6-ß-naltrexol are 4 hours and 13 hours, respectively. Naltrexone and 6-ß-naltrexol are dose proportional in terms of AUC and Cmax over the range of 50 to 200 mg and do not accumulate after 100 mg daily doses.


This medication is used to prevent people who have been addicted to certain drugs (opiates) from taking them again. It is used as part of a complete treatment program for drug abuse (e.g., compliance monitoring, counseling, behavioral contract, lifestyle changes). This medication must not be used in people currently taking opiates, including methadone. Doing so can cause sudden withdrawal symptoms.

Naltrexone belongs to a class of drugs known as opiate antagonists. It works in the brain to prevent opiate effects (e.g., feelings of well-being, pain relief). It also decreases the desire to take opiates.

This medication is also used to treat alcohol abuse. It can help people drink less alcohol or stop drinking altogether. It also decreases the desire to drink alcohol when used with a treatment program that includes counseling, support, and lifestyle changes.

How to use naltrexone

Take this medication by mouth with or without food, usually 50 milligrams once daily or as directed by your doctor. This medication may be given as part of a program where a health care professional will watch you take the medication. In this case, your doctor may order a higher dose (100-150 milligrams) to be taken every 2-3 days to make it easier to schedule clinic visits. Naltrexone may be taken with food or antacids if stomach upset occurs.

A urine test should be done to check for recent opiate drug use. Your doctor may give you another medication (naloxone challenge test) to check for opiate use. Do not use any opiates for at least 7 days before starting naltrexone. You may need to stop certain opiate drugs (such as methadone) 10 to 14 days before starting naltrexone.

Dosage is based on your medical condition and response to treatment. Your doctor may start you at a lower dose and monitor you for any side effects or withdrawal symptoms before increasing your dose. Take this medication as directed. Do not increase your dose, take it more often, or stop taking it without your doctor's approval. 

Use this medication regularly to get the most benefit from it. To help you remember, take it at the same time each day.

Tell your doctor if you start using drugs or alcohol again.


Following oral administration, Naltrexone undergoes rapid and nearly complete absorption with approximately 96% of the dose absorbed from the gastrointestinal tract. Peak plasma levels of both Naltrexone and 6-ß-naltrexol occur within one hour of dosing.


The volume of distribution for Naltrexone following intravenous administration is estimated to be 1350 liters. In vitro tests with human plasma show Naltrexone to be 21% bound to plasma proteins over the therapeutic dose range.


The systemic clearance (after intravenous administration) of Naltrexone is ~ 3.5 L/min, which exceeds liver blood flow (~ 1.2 L/min). This suggests both that Naltrexone is a highly extracted drug (>98% metabolized) and that extra-hepatic sites of drug metabolism exist. The major metabolite of Naltrexone is 6-ß-naltrexol. Two other minor metabolites are 2-hydroxy-3-methoxy-6-ß-naltrexol and 2-hydroxy-3-methyl-Naltrexone. Naltrexone and its metabolites are also conjugated to form additional metabolic products.


The renal clearance for Naltrexone ranges from 30 to 127 mL/min and suggests that renal elimination is primarily by glomerular filtration. In comparison the renal clearance for 6-ß-naltrexol ranges from 230 to 369 mL/min, suggesting an additional renal tubular secretory mechanism. The urinary excretion of unchanged Naltrexone accounts for less than 2% of an oral dose; urinary excretion of unchanged and conjugated 6-ß-naltrexol accounts for 43% of an oral dose. The pharmacokinetic profile of Naltrexone suggests that Naltrexone and its metabolites may undergo enterohepatic recycling.

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