nalxone hydrochloride dihydrate
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naloxone hydrochloride dihydrate pharmacology mechanism dose clinical use
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"naloxone" AND "opioid overdose"
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Naloxone Hydrochloride Dihydrate
Chemical Identity
Naloxone hydrochloride dihydrate is the pharmaceutical salt form of naloxone used in most injectable formulations. Its full chemical name is 4,5α-epoxy-3,14-dihydroxy-17-(prop-2-enyl)morphinan-6-one hydrochloride dihydrate.
| Property | Value |
|---|---|
| Molecular formula | C₁₉H₂₁NO₄ · HCl · 2H₂O |
| Molecular weight | 399.9 g/mol (anhydrous HCl salt: 363.84 g/mol) |
| Appearance | White to off-white crystalline powder |
| Solubility | Freely soluble in water and dilute acids; slightly soluble in alcohol; practically insoluble in ether and chloroform |
| pH of injection | 3.1 - 4.5 |
| Structure class | Synthetic congener of oxymorphone (phenanthrene morphinan) |
The "dihydrate" refers to two molecules of water of crystallization included in the salt. In parenteral formulations, a dose is expressed as the HCl salt equivalent - e.g., 400 micrograms/mL solution contains 400 mcg of naloxone hydrochloride as the dihydrate form. The key structural difference from oxymorphone is replacement of the N-methyl group with an N-allyl (prop-2-enyl) group, which confers antagonist rather than agonist activity.
Mechanism of Action
Naloxone is a pure competitive opioid receptor antagonist. It competes with opioids for the same receptor binding sites - with highest affinity for the μ (mu) receptor, and also binding κ (kappa) and σ (sigma) receptors. By occupying these receptors without activating them, it fully reverses all opioid-mediated effects:
- Respiratory depression
- Sedation / CNS depression
- Hypotension
- Miosis
- Nausea, vomiting, pruritus, urinary retention, biliary spasm
- Psychotomimetic and dysphoric effects of partial agonist-antagonists (e.g., pentazocine, butorphanol, nalbuphine)
Importantly, naloxone has no intrinsic agonist activity at standard doses - when given to an opioid-naive patient, it produces no significant pharmacological effect. This distinguishes it from earlier antagonists like nalorphine and levallorphan, which had unacceptable partial agonist side effects.
Paradoxical low-dose effect: At very low doses (<25 mcg IV), naloxone may paradoxically enhance analgesia by blocking presynaptic autoinhibition and promoting endogenous opioid release. This has been exploited in low-dose naloxone infusions (0.25 mcg/kg/hr) to reduce opioid consumption without reversing analgesia. At doses above this threshold, antagonism predominates. - Miller's Anesthesia, 10e
Pharmacokinetics
| Parameter | Detail |
|---|---|
| Onset (IV) | 1-2 minutes |
| Onset (IM/SC) | 5-6 minutes |
| Onset (IN) | 6-8 minutes |
| Half-life | ~60-90 minutes |
| Duration of action | 20-90 minutes (shorter when a large opioid load is present) |
| Oral bioavailability | Very poor (extensive first-pass hepatic metabolism) |
| Metabolism | Liver - primarily glucuronide conjugation (naloxone-3-glucuronide) |
| Excretion | Urine |
The short duration of action is the most clinically important pharmacokinetic feature: naloxone may wear off before the opioid does, causing "renarcotization." Patients must be closely monitored after treatment. This is especially relevant for long-acting opioids (methadone, sustained-release morphine) and high-potency synthetic opioids (fentanyl, nitazenes, buprenorphine). - Tintinalli's Emergency Medicine and Miller's Anesthesia, 10e
Intranasal naloxone provides a pharmacokinetic profile similar to IM/SC and is widely used in bystander programs due to ease of administration. - Tintinalli's Emergency Medicine
Indications
- Opioid overdose - complete or partial reversal of respiratory depression, sedation, hypotension from natural and synthetic opioids (morphine, heroin, fentanyl, methadone, oxycodone, hydromorphone, propoxyphene, buprenorphine, pentazocine, butorphanol, nalbuphine, cyclazocine)
- Postoperative opioid reversal - reversal of residual opioid anesthesia
- Diagnosis of suspected acute opioid overdose
- Adjunctive therapy in septic shock - may help increase blood pressure in refractory cases
- Take-home naloxone programs for opioid-dependent patients and their carers
Dosing (Adults)
| Route | Indication | Dose |
|---|---|---|
| IV | Opioid-dependent, modest respiratory depression | 0.04 mg (start low to avoid precipitating withdrawal) |
| IV | Opioid-naive, modest depression | 0.4 mg |
| IV | Apnea / cyanosis / severe depression | 2 mg - repeat every 3 min up to 10 mg |
| IM or SC | Any | 2 mg |
| Intranasal | Any | 2 mg (1 mg per nostril) |
| Nebulized (inhaled) | Any | 2 mg in 3 mL normal saline |
| Postoperative (IV) | Titrated reversal | 0.005-0.01 mg increments every 2-3 min |
Repeat doses every 2-3 minutes as needed. Duration depends on opioid load present. - Tintinalli's Emergency Medicine, Table 186-3
Pediatric: Initial IV dose 0.01 mg/kg; if inadequate, escalate to 0.1 mg/kg.
Buprenorphine/high-affinity opioids: Larger doses and/or continuous IV infusion of naloxone may be required because buprenorphine's high receptor affinity and slow dissociation limit reversal. Respiratory depression from buprenorphine can outlast naloxone boluses. - Miller's Anesthesia, 10e
Opioid Overdose Response Protocol
Figure: WHO protocol for lay naloxone use - Maudsley Prescribing Guidelines, 15e
Adverse Effects and Cautions
- Acute opioid withdrawal - the main risk; presents as agitation, tachycardia, hypertension, sweating, vomiting, pulmonary edema. Use low starting doses in opioid-dependent patients.
- Cardiovascular stimulation - increases in heart rate and blood pressure post-reversal due to pain, rapid awakening, sympathetic activation, and hypercapnia-driven sympathetics. Most dangerous in patients with underlying coronary artery disease.
- Pulmonary edema - rare but serious complication.
- Re-narcotization - occurs when naloxone wears off before the opioid; mandates monitoring for at least 2-3 hours (longer for sustained-release preparations or methadone).
- Physical dependence - naloxone produces no tolerance or dependence itself, but in physically dependent patients will precipitate withdrawal.
- Not effective against non-opioid respiratory depressants (benzodiazepines, barbiturates, alcohol).
Formulations Available
- Injection (standard): 0.4 mg/mL in 1 mL ampoules (most common)
- High-dose injection: 10 mg/0.4 mL auto-injector (for high-potency fentanyl analogue exposure, approved for ages ≥12 years)
- Intranasal spray: 2 mg/0.1 mL (Narcan nasal spray) - first generic approved by FDA
- Combination product: buprenorphine/naloxone sublingual (Suboxone) - naloxone included to deter injection misuse
Recent Evidence
A 2025 systematic review and meta-analysis (PMID 40133970) confirmed that community naloxone distribution programs significantly reduce opioid overdose deaths among people who use drugs - supporting broad take-home naloxone access policies. A 2024 systematic review (PMID 38563492) explored healthcare professionals' perceptions of take-home naloxone in acute care settings.
Sources: Miller's Anesthesia 10e | Tintinalli's Emergency Medicine | Maudsley Prescribing Guidelines 15e | FDA prescribing information | Health Canada product monograph
what is it used for?
Based on the detailed information already in our conversation, here is a focused summary of what naloxone hydrochloride dihydrate is used for:
What Naloxone Hydrochloride Dihydrate Is Used For
1. Opioid Overdose Reversal (Primary Use)
The main use is emergency reversal of opioid overdose. It rapidly reverses:
- Respiratory depression (the main killer in overdose)
- Sedation / loss of consciousness
- Hypotension
- Miosis (pinpoint pupils)
It works against all opioids - morphine, heroin, fentanyl, methadone, oxycodone, hydromorphone, buprenorphine, codeine, tramadol, and synthetic opioids.
2. Postoperative Opioid Reversal
Used in the recovery room to reverse residual opioid anesthesia after surgery - titrated carefully in small IV increments (0.005-0.01 mg) to restore breathing without completely abolishing pain relief.
3. Diagnosis of Suspected Opioid Overdose
If a patient is unconscious and opioid overdose is suspected, giving naloxone is both therapeutic and diagnostic - if the patient wakes up and breathing improves, opioids were the cause.
4. Take-Home / Community Overdose Programs
Dispensed to opioid-dependent patients, their family members, carers, and first responders as a life-saving kit for use before emergency services arrive. Available as nasal spray or pre-filled syringe for lay use.
5. Reversal of Mixed Agonist-Antagonists
Also reverses psychotomimetic and dysphoric effects of partial opioid agonists such as pentazocine, butorphanol, and nalbuphine.
6. Adjunct in Septic Shock
Used off-label as an adjunctive agent to raise blood pressure in refractory septic shock - the mechanism is thought to involve reversal of endogenous opioid-mediated vasodilation.
7. Component of Buprenorphine/Naloxone (Suboxone)
Naloxone is included in sublingual buprenorphine tablets not for its therapeutic effect (it has negligible sublingual absorption), but to deter injection misuse - if injected, the naloxone component would precipitate immediate withdrawal.
What It Does NOT Treat
- Overdose from benzodiazepines, alcohol, or barbiturates - naloxone has no effect on non-opioid CNS depressants
- Pain - it has no analgesic properties of its own
The single most important clinical point is its short duration of action (20-90 minutes), which is often shorter than the opioid it reverses. Patients must be monitored closely for re-narcotization (the opioid effect returning as the naloxone wears off).
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