1. Clinical Overview
Naltrexone is a potent, long-acting, competitive opioid receptor antagonist. It is a synthetic congener of oxymorphone with no intrinsic agonist activity. It is primarily used for the management of alcohol dependence and opioid dependence by blocking the euphoric and sedative effects of opioids and reducing the craving for alcohol. In the Indian context, it is a Schedule H drug, widely prescribed as part of comprehensive addiction treatment programs.
| Onset | Duration | Bioavailability |
|---|---|---|
| Oral: Peak plasma concentrations in 60 minutes. Opioid blockade effect begins within 30-60 minutes. | Approximately 24-72 hours for opioid blockade, depending on dose. A single 50mg dose blocks the pharmacologic effects of 25mg IV heroin for up to 24 hours. | Oral: 5-40% (mean ~20-30%) due to significant first-pass metabolism. |
2. Mechanism of Action
Naltrexone is a pure opioid antagonist with highest affinity for mu-opioid receptors. It competitively binds to and blocks these receptors in the central nervous system, preventing exogenously administered opioids (like heroin, morphine) from producing euphoria and sedation. In alcohol dependence, its mechanism is complex and not fully elucidated. It is believed to modulate the mesolimbic dopaminergic pathway (reward pathway) by blocking endogenous opioid peptides (endorphins) released by alcohol consumption, thereby reducing the reinforcing effects and craving for alcohol.
3. Indications & Uses
- Maintenance treatment of opioid dependence (post-detoxification) to prevent relapse.
- Adjunct in the management of alcohol dependence to reduce craving and maintain abstinence.
4. Dosage & Administration
Adult Dosage: **Opioid Dependence:** Initiate only after 7-10 days of opioid-free period. 25mg initially; if no withdrawal signs, 50mg once daily thereafter. **Alcohol Dependence:** 50mg once daily. Can be initiated without a washout period. Therapy should be part of a comprehensive treatment program.
Administration: Administer orally with or without food. Taking with food may reduce GI upset. Tablet can be crushed if needed. Should be taken at the same time each day. For patients on 50mg/day, missed dose: Take as soon as remembered, but skip if close to next dose. Do not double dose.
5. Side Effects
Common side effects may include:
- Nausea (10%)
- Abdominal pain/cramps
- Headache
- Fatigue
- Anxiety, nervousness
- Insomnia
- Joint/muscle pain
6. Drug Interactions
| Drug | Effect | Severity |
|---|---|---|
| Opioid Analgesics (Morphine, Codeine, Tramadol, Dextropropoxyphene) | Naltrexone will block analgesic and euphoric effects. May require significantly higher, dangerous doses of opioid to overcome blockade, leading to potential overdose/ respiratory depression if naltrexone effect wears off. | Major |
| Opioid-containing Cough Syrups (Pholcodine, Codeine) | Reduced antitussive effect. | Moderate |
| Thioridazine | Reported increased lethargy and somnolence. | Moderate |
| Disulfiram | Co-administration in alcohol dependence may increase risk of hepatotoxicity. Monitor LFTs closely. | Major |
| Acamprosate | Can be used safely in combination for alcohol dependence. No significant interaction. | Minor |
7. Patient Counselling
- **DO** carry identification stating you are on naltrexone.
- **DO** inform all healthcare providers (including dentists) that you are taking naltrexone before any treatment.
- **DO** attend all counseling and support sessions as part of your treatment program.
- **DO NOT** take any opioid-containing medicines (painkillers, cough syrups). They will not work and attempting to overcome the blockade can be fatal.
- **DO NOT** start naltrexone if you have used any opioids in the last 7-10 days.
8. Toxicology & Storage
Overdose: No classic opioid overdose syndrome. Symptoms may include: Nausea, vomiting, abdominal cramps, drowsiness, dizziness, lethargy, injection site reactions (for depot), and at very high doses (>800mg/day), evidence of hepatotoxicity (elevated LFTs).
Storage: Store at room temperature (15-30°C). Protect from light and moisture. Keep in the original blister pack or container. Keep out of reach of children.