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Mixed opioid agonist-antagonist Pregnancy: SPC section 4.6: regular use during pregnancy may cause drug dependence in the foetus, leading to withdrawal symptoms in the neonate. If opioid use is required for a prolonged period in a pregnant woman, advise the patient of the risk of neonatal opioid withdrawal syndrome and ensure that appropriate treatment will be available. Administration during labour may depress respiration in the neonate and an antidote for the child should be readily available. Breast feeding: administration to nursing women is not recommended as pentazocine may be secreted in breast milk and may cause respiratory depression in the infant.

Pentazocine

Brand names: Fortral (largely discontinued UK)

Pentazocine is an opioid analgesic used for moderate-to-severe pain.

Adult dose

Dose: Adults - relief of moderate to severe pain (each capsule contains pentazocine hydrochloride 50 mg): the usual initial dose is 50 mg every four hours after meals, followed by one to two 50 mg capsules every three to four hours; the dosage is usually tailored to the individual patient and to the severity of the pain
Route: Oral (capsules)
Frequency: Initially every 4 hours after meals; then one to two 50 mg capsules every 3 to 4 hours
Max: 600 mg daily
The pinned product is PENTAZOCINE CAPSULES 50 MG (PL113110530, Tillomed Laboratories Limited); section 4.1: 'a strong analgesic for the relief of moderate to severe pain'; section 4.2: 'For oral use'. NOT COVERED BY THIS PRODUCT: intramuscular, intravenous and subcutaneous routes - no injectable dose is written. The label prints the maximum as 'The maximum dose is 600mg daily' (section 4.2, Adults). Elderly: 'Since impaired renal or hepatic function is often associated with ageing, elderly patients may require smaller doses of pentazocine' (no figure printed). Children: 'Not recommended for children under 12 years of age. It is recommended that another dosage form i.e. tablets, be used as appropriate for this age group.' The SPC prints no paediatric dose, so paedDose is null. Section 4.2: before starting treatment with opioids, hold a discussion with the patient to put in place a strategy for ending treatment with pentazocine, to minimise the risk of addiction and drug withdrawal syndrome. Section 4.4: after long term treatment (> 3 months) with analgesics used every second day or more frequently, medication-overuse headache may develop - it should not be treated by increasing the dose; concomitant benzodiazepines or related sedatives should be reserved for patients without alternatives, at the lowest effective dose for as short a duration as possible. The capsules contain lactose (84.8 mg per capsule).

Dose adjustments

Renal

SPC section 4.4: caution in patients with severe renal impairment, as it may increase the bioavailability of pentazocine and call for a reduction in dosage; section 4.2: elderly patients (often with impaired renal or hepatic function) may require smaller doses. No reduced figure is printed.

Hepatic

SPC section 4.4: caution in patients with severe hepatic impairment, as it may increase the bioavailability of pentazocine and call for a reduction in dosage. No reduced figure is printed.

Dose independently re-sourced from UK Summary of Product Characteristics (SPC) via MHRA Products — clinician-reviewed. Always confirm against the product SmPC and your local formulary before prescribing.

Contraindications

  • Hypersensitivity to the active substance or to any of the excipients
  • Established respiratory depression, especially in the presence of cyanosis and excessive bronchial secretion
  • Acute alcoholism
  • Acute bronchial asthma
  • Heart failure secondary to chronic lung disease
  • Porphyria
  • Head injuries or conditions in which intracranial pressure is raised, or pathological brain conditions where clouding of the sensorium is undesirable

Side effects

  • Most frequent: light-headedness, dizziness, nausea, vomiting and sweating; sedation is the most common side effect (less than with morphine)
  • Cardiac and vascular: tachycardia, bradycardia, palpitations, transient hypertension, hypotension, circulatory depression
  • Nervous system: hallucinations (occasionally), dysphoria, headache, disorientation, mood changes, nightmares, insomnia, paraesthesia, syncope, euphoria, grand mal convulsions, raised intracranial pressure, confusion, muscle tremor, thought disturbances
  • Respiratory depression
  • Gastrointestinal: constipation, dry mouth, biliary tract spasm
  • Blood: transient eosinophilia, agranulocytosis, depression of white blood cell count
  • Skin and immune: facial oedema, flushing, rashes, urticaria, dermatitis including pruritus, chills, allergic reactions; toxic epidermal necrolysis
  • Urinary retention, ureteric tract spasm; alterations in rate or strength of uterine contractions; decreased libido or potency; hypothermia; miosis and disturbances of vision
  • Drug dependence (prolonged use of high dosage may produce dependence) and drug withdrawal syndrome (uncommon)

Interactions

  • Monoamine oxidase inhibitors - may enhance the opioid effects of pentazocine; do not use in patients receiving MAOIs or who have received them within the past 14 days
  • Phenothiazines, tricyclic antidepressants and ethyl alcohol - enhance the central depressant effects of pentazocine (opposed by respiratory stimulants such as doxapram)
  • Tobacco smoking - appears to enhance the metabolic clearance rate of pentazocine, reducing the clinical effectiveness of a standard dose
  • Stronger opioid agonists such as diamorphine and morphine - pentazocine can antagonise their effects and may provoke withdrawal symptoms if given to narcotic addicts; pentazocine is itself antagonised by naloxone
  • Benzodiazepines or related sedatives - increased risk of sedation, respiratory depression, coma and death; limit dose and duration of concomitant use

Monitoring

  • Patients should be closely monitored for signs of misuse, abuse or addiction; review the clinical need for analgesic treatment regularly
  • Signs of tolerance: treatment less effective with chronic use and a need to increase the dose; take care to avoid unnecessary increase in dosage in chronic use
  • Drug withdrawal syndrome on abrupt cessation or dose reduction - taper the dose gradually when therapy is no longer required
  • Pentazocine can both depress and elevate blood pressure: particular caution in phaeochromocytoma, in the acute phase following myocardial infarction, and where alterations of vascular resistance and blood pressure would be undesirable
  • Caution in patients prone to seizures, previously on large doses of narcotics, opioid-dependent, hypothyroid, with adrenocortical insufficiency or prostatic hypertrophy, inflammatory or obstructive bowel disorders, cholecystitis, pancreatitis or other unidentified abdominal pain
  • Concomitant sedatives: follow patients closely for signs and symptoms of respiratory depression and sedation

Clinical monograph

How it works

It acts as a kappa-opioid receptor agonist with partial agonist or weak antagonist activity at the mu-opioid receptor.

Prescribing in practice

  • As a mixed agonist–antagonist it can precipitate withdrawal in patients dependent on full mu-opioid agonists and may cause respiratory depression; avoid combining with other opioids.
  • It can cause psychotomimetic effects such as hallucinations, dysphoria and confusion, and carries a risk of dependence and misuse.
  • Use with caution in respiratory impairment, head injury, hepatic or renal impairment, and where it may raise cardiac workload.

Monitoring

Monitor pain control, sedation, respiratory and mental status, and watch for signs of dependence or withdrawal.

Counselling the patient

  • This medicine can cause drowsiness; do not drive or operate machinery if affected.
  • Avoid alcohol and tell your clinician about any other opioid medicines you take.
  • Report hallucinations, severe drowsiness or difficulty breathing.

Evidence & guidelines

Pentazocine is a long-established opioid analgesic, though newer agents are often preferred owing to its psychotomimetic effects.

Reference: FPM Opioids Aware; NICE NG193; Drug verified in RxNorm (NLM); confirm dosing against the manufacturer SPC (eMC). Verify against your local formulary and current prescribing references before prescribing. The structured dose values shown have been reviewed by a clinician. Monograph status: clinician-reviewed (2026-07-04).

Related

Curated clinical cross-links plus same-class fallbacks.

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