Pentazocine is a benzomorphan derivative with mixed opioid agonist and antagonist actions. It alters perception of and response to pain and produces generalised CNS depression by binding to opiate receptors in the CNS and acting as a partial agonist/antagonist.

Onset
Oral, IM, SC: 15-30 minutes; IV 2-3 minutes.

Duration
Oral: 4-5 hours; parenteral: 2-3 hours.

Absorption
Absorbed from the GI tract (oral), through the muscles (IM); peak plasma concentrations after 1-3 hours (oral), 15 minutes-1 hour (IM). Oral bioavailability: approx 20% (60-70% in cirrhosis).

Distribution
Crosses the placenta. Protein-binding: 60%

Metabolism
Hepatic; via oxidative and glucuronide conjugation pathways; extensive 1st pass.

Excretion
Elimination half life: 2-3 hours; prolonged in hepatic impairment; excretion via urine (small amounts as unchanged drug).

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Physical dependence; sedation, dizziness, euphoria, lightheadedness, alterations of mood; respiratory depression; visual hallucinations, disorientation, confusion; hypertension, tachycardia, circulatory depression; shock; hypotension; nausea, vomiting, constipation; seizures, diaphoresis; rash; blood dyscrasias; local tissue damages (SC), muscle fibrosis (IM).

Potentially Fatal: Respiratory depression, hypotension, circulatory failure, deepening coma, convulsions.

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May precipitate withdrawal in narcotic addicts. Impaired respiratory, renal and hepatic function; morbidly obese patients; thyroid dysfunction; prostatic hyperplasia or urinary stricture; biliary tract impairment; adrenal insufficiency (including Addison's disease); abdominal conditions. Elderly or debilitated patients; seizure-prone patients; children and infants (safety and efficacy not established in <1 year); lactation. May impair ability to drive or operate machinery. Administer IM rather than SC (when frequent injections are needed) and injection sites should be varied.

Depressant affects potentiated by alcohol, CNS depressants; concurrent use with fluoxetine may lead to diaphoresis, ataxia flushing and tremor associated with serotonin syndrome.

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Oral
Moderate to severe pain
Adult: 50-100 mg every 3-4 hours; max 600 mg daily.
Child: 6-12 years: As hydrochloride: 25 mg every 3-4 hours.

CrCl (ml/min)    Dosage Recommendation
10-50               75% normal dose.
<10                  50% normal dose.
Hepatic impairment: Reduce dose or avoid in liver disease.

Parenteral
Moderate to severe pain
Adult: 30-60 mg SC, IM or IV injection given every 3-4 hours.
Child: >1 year: up to 1 mg/kg SC, IM; up to 500 mcg/kg IV every 3-4 hours.

CrCl (ml/min)    Dosage Recommendation
10-50               75% normal dose.
<10                  50% normal dose.
Hepatic impairment: Reduce dose or avoid in liver disease.

Rectal
Moderate to severe pain
Adult: 50 mg suppository up to 4 times daily.
Child: Not recommended.

CrCl (ml/min)    Dosage Recommendation
10-50               75% normal dose
< 10                 50% normal dose
Hepatic impairment: Reduce dose or avoid in liver disease.

Incompatibility
At Y site: Nafcillin; when admixed: aminophylline, amobarbital, pentobarbital, phenobarbital, sodium bicarbonate; in syringe: glycopyrrolate, heparin, phenobarbital.

Should be taken with food. (Take after meals.)

Contraindicated in pregnancy

Category C: Either studies in animals have revealed adverse effects on the foetus (teratogenic or embryocidal or other) and there are no controlled studies in women or studies in women and animals are not available. Drugs should be given only if the potential benefit justifies the potential risk to the foetus.

If used for prolonged periods or in high doses at term:

Category D: There is positive evidence of human foetal risk, but the benefits from use in pregnant women may be acceptable despite the risk (e.g., if the drug is needed in a life-threatening situation or for a serious disease for which safer drugs cannot be used or are ineffective).

Caution when used during lactation

Not recommended for children younger than 12 years of age.

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Head injury; narcotic dependence; respiratory depression; raised intracranial pressure; MI; heart failure; arterial or pulmonary hypertension; porphyria; pregnancy (prolonged use or high doses at term).

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