Unit
- Mechanism of action: Naltrexone is an opioid receptor antagonist that competitively blocks mu-opioid receptors, reducing the reinforcing effects of alcohol and opioids. This blockade diminishes cravings and prevents euphoria from substance use, supporting abstinence-focused treatment. The binding is reversible and dose-dependent.
- Pharmacokinetics: Oral naltrexone is rapidly absorbed with peak plasma concentrations reached in about one hour. It undergoes extensive first-pass metabolism to 6-beta-naltrexol, an active metabolite. The half-life of naltrexone is approximately four hours, while 6-beta-naltrexol has a longer half-life of 13 hours.
- Dosage forms: Available as oral tablets (50 mg daily) and extended-release injectable suspension (380 mg intramuscularly every four weeks). The injectable formulation improves adherence by reducing dosing frequency. Both forms require a prescription and are initiated under medical supervision.
- Drug interactions: Concurrent use with opioid analgesics can lead to opioid withdrawal or reduced analgesic effect. Caution is needed when patients require opioid pain management. Naltrexone does not interact significantly with non-opioid medications, but liver enzyme impacts from other drugs should be monitored.
- Hepatic metabolism: Naltrexone is primarily metabolized by the liver via dihydrodiol dehydrogenase. Patients with hepatic impairment may have reduced clearance, requiring dose adjustment or alternative therapies. Regular liver function tests are recommended during treatment.
- Onset and duration: Oral naltrexone reaches therapeutic effect within hours and maintains receptor blockade for up to 24 hours, making daily dosing sufficient. The injectable form provides continuous blockade for one month, with a gradual decline in drug levels afterward.
- Receptor selectivity: Naltrexone has high affinity for mu-opioid receptors and moderate affinity for kappa and delta receptors. This selectivity underlies its efficacy in reducing cravings while minimizing side effects compared to non-selective antagonists.
- Clinical pharmacology considerations: Individual variability in metabolism and receptor density can affect response. Genetic polymorphisms in the mu-opioid receptor gene (OPRM1) may influence treatment outcomes, though routine genetic testing is not yet standard practice.
- Contraindications: Naltrexone is contraindicated in patients currently using opioid analgesics or in acute opioid withdrawal. It is also contraindicated in individuals with severe hepatic impairment, acute hepatitis, or liver failure. A negative opioid screen is typically required before initiation to avoid precipitating withdrawal symptoms.
- Common adverse effects: Patients frequently report nausea, headache, dizziness, fatigue, and insomnia during the first few weeks of treatment. These effects are usually mild to moderate and often resolve with continued use. Gastrointestinal symptoms can be minimized by taking oral naltrexone with food or gradually titrating the dose.
Scope of Practice
Role: Clinical Support Staff in Naltrexone Therapy
Within scope:
- Educating patients on how naltrexone works and what to expect during treatment.
- Assisting with medication administration under supervision (oral dosing) or preparing injection sites.
- Monitoring and documenting patient-reported side effects and cravings.
- Reinforcing the importance of adherence to dosing schedules.
Outside scope:
- Prescribing or adjusting naltrexone dosage independently.
- Interpreting pharmacokinetic lab results for medical decision-making.
- Counseling on drug interactions without consulting the prescriber.
- Administering injectable formulations without training and supervision.
Quick Revision
- Naltrexone is an opioid receptor antagonist that blocks mu-opioid receptors to reduce cravings.
- Oral and injectable formulations offer dosing flexibility, with the injectable improving adherence.
- Metabolism occurs in the liver; hepatic function should be monitored.
- Support staff can educate and monitor but cannot prescribe or adjust doses.