Mechanism of action
Pethidine is a synthetic phenylpiperidine opioid that acts primarily as an agonist at μ-opioid receptors in the central nervous system and peripheral tissues. Activation of these receptors inhibits ascending pain pathways, alters pain perception, and increases pain tolerance.
Unlike morphine, pethidine has significant antimuscarinic (anticholinergic) properties, which can cause tachycardia, dry mouth, and mydriasis. It also possesses weak local anesthetic activity due to sodium channel blockade. Pethidine has approximately one-tenth the analgesic potency of morphine.
The drug produces typical opioid effects including analgesia, euphoria, respiratory depression, miosis (though less than morphine due to anticholinergic effects), decreased gastrointestinal motility, and physical dependence with chronic use. Its intermediate lipid solubility allows moderate CNS penetration.
- Pethidine is metabolized to norpethidine, a neurotoxic metabolite with a 15-40 hour half-life that accumulates in renal impairment
- Norpethidine causes tremors, myoclonus, and seizures; naloxone does NOT reverse norpethidine toxicity and may worsen seizures
- Absolute contraindication with MAOIs (within 14 days): risk of fatal serotonin syndrome and hypertensive crisis
- Pethidine has anticholinergic effects causing tachycardia (unlike other opioids that cause bradycardia) and less miosis
- Limit use to 48-72 hours maximum; never use for chronic pain due to norpethidine accumulation
- Pethidine has 1/10th the analgesic potency of morphine with shorter duration of action (2-4 hours)
- Renal impairment is a major risk factor for seizures; avoid or use with extreme caution in reduced renal function
- Has declined in clinical use; many hospitals have removed it from formularies due to unique toxicity profile
Indications
Pethidine is used for:
- Acute moderate to severe pain (postoperative, trauma, medical conditions)
- Obstetric analgesia during labor (though use has declined)
- Perioperative analgesia as part of balanced anesthesia
- Premedication before anesthesia or procedures
- Rigors associated with amphotericin B or postoperative shivering
Pethidine use has significantly declined in many settings due to concerns about its toxic metabolite norpethidine. Many institutions have removed it from formularies or restricted its use to short-term management only. Morphine, fentanyl, and other opioids are generally preferred for most indications. Pethidine should not be used for chronic pain management.
Pharmacokinetics
Absorption and distribution: Pethidine is well absorbed after oral, intramuscular, or subcutaneous administration, though oral bioavailability is only 40-60% due to significant first-pass metabolism. Peak plasma concentrations occur 1-2 hours after oral dosing and 30-50 minutes after intramuscular injection. The drug is 60-80% protein bound and has a volume of distribution of 3-5 L/kg.
Metabolism: Pethidine undergoes extensive hepatic metabolism via N-demethylation to norpethidine (normeperidine), an active metabolite with half the analgesic potency but twice the CNS excitatory effects of the parent drug. Norpethidine has a long half-life (15-40 hours, compared to 3-5 hours for pethidine) and accumulates with repeated dosing or in renal impairment. Other metabolites include pethidinic acid and normepetidinic acid.
Excretion: Metabolites are primarily excreted in urine. Renal impairment significantly prolongs norpethidine elimination, increasing seizure risk. The elimination half-life of pethidine is 3-5 hours in adults with normal renal function but can be prolonged in elderly patients and those with hepatic or renal disease.
Special populations: Elderly patients and those with renal or hepatic impairment require dose reduction and careful monitoring. Pethidine crosses the placenta and enters breast milk.
Adverse effects
Common adverse effects
- CNS: Sedation, dizziness, euphoria, dysphoria, confusion
- Cardiovascular: Tachycardia (due to anticholinergic effects), hypotension (especially orthostatic)
- Respiratory: Dose-dependent respiratory depression, decreased cough reflex
- Gastrointestinal: Nausea, vomiting, constipation (less than morphine), dry mouth
- Genitourinary: Urinary retention, increased bladder sphincter tone
- Dermatologic: Pruritus, sweating, flushing
Serious adverse effects
Norpethidine toxicity is the most important serious adverse effect. Norpethidine is a CNS stimulant that can cause:
- Tremors, myoclonus, muscle twitching
- Hyperreflexia and hyperexcitability
- Seizures (generalized tonic-clonic)
- Anxiety, agitation, mood changes
Risk factors include repeated dosing, high cumulative doses, renal impairment, elderly age, and concurrent use with enzyme inhibitors. Naloxone does not reverse norpethidine-induced seizures, and may even worsen them.
Other serious effects include:
- Severe respiratory depression and apnea
- Severe hypotension and syncope
- Serotonin syndrome when combined with MAOIs or other serotonergic drugs (potentially fatal)
- Anaphylaxis and histamine release (local reactions at injection sites)
- Physical dependence and withdrawal syndrome with chronic use
Black box warnings
Pethidine carries warnings regarding:
- Risk of addiction, abuse, and misuse
- Life-threatening respiratory depression
- Accidental ingestion (especially in children)
- Neonatal opioid withdrawal syndrome with prolonged maternal use
- Risks of concomitant use with benzodiazepines or other CNS depressants
Contraindications and warnings
Absolute contraindications
- Concurrent or recent MAOI use (within 14 days): risk of severe, potentially fatal reactions including serotonin syndrome, hypertension, hyperpyrexia, seizures, and coma
- Known hypersensitivity to pethidine or phenylpiperidine derivatives
- Acute or severe bronchial asthma in unmonitored settings
- Significant respiratory depression
- Acute or severe hepatic impairment
- Paralytic ileus or gastrointestinal obstruction
Relative contraindications and warnings
- Renal impairment: High risk of norpethidine accumulation and seizures; avoid or use extreme caution with dose reduction
- Hepatic impairment: Reduced metabolism and increased effects
- Elderly patients: Increased sensitivity and risk of toxicity
- Head injury or increased intracranial pressure (may obscure neurological assessment)
- Seizure disorders (norpethidine lowers seizure threshold)
- Chronic obstructive pulmonary disease or cor pulmonale
- Hypothyroidism, Addison disease, prostatic hypertrophy
- Biliary tract disease (may cause sphincter of Oddi spasm)
Pregnancy and lactation
Pethidine is FDA pregnancy category C (now replaced by PHLR labeling). It crosses the placenta and can cause neonatal respiratory depression and withdrawal. Chronic use during pregnancy may lead to neonatal opioid withdrawal syndrome. If used during labor, have naloxone available for neonatal resuscitation.
Pethidine enters breast milk and may cause sedation or respiratory depression in nursing infants. The American Academy of Pediatrics considers it compatible with breastfeeding when used occasionally in low doses, but prolonged or repeated use is not recommended.
Drug interactions
Major interactions
MAOIs: Life-threatening interaction producing serotonin syndrome, severe hypertension or hypotension, respiratory depression, coma, and seizures. Absolutely contraindicated within 14 days of MAOI use.
Other serotonergic drugs: SSRIs, SNRIs, TCAs, triptans, linezolid, methylene blue — increased risk of serotonin syndrome. Use with extreme caution and monitor closely.
CNS depressants: Benzodiazepines, barbiturates, alcohol, antihistamines, other opioids, general anesthetics — additive respiratory depression, sedation, hypotension, coma, and death. Avoid combination or reduce doses significantly.
Phenytoin: Increases pethidine metabolism, reducing analgesic effect but potentially increasing norpethidine formation.
Ritonavir and protease inhibitors: May alter pethidine metabolism unpredictably.
Naloxone and naltrexone: Opioid antagonists reverse analgesic effects and can precipitate acute withdrawal. Naloxone does not reverse and may worsen norpethidine-induced seizures.
Moderate interactions
- Anticholinergics: Additive anticholinergic effects (dry mouth, urinary retention, constipation, confusion)
- Cimetidine: May reduce pethidine metabolism, increasing plasma concentrations
- Acyclovir: Potential for increased norpethidine neurotoxicity
- Diuretics: Opioids may reduce diuretic efficacy
- Muscle relaxants: Enhanced neuromuscular blocking effects
Patients should avoid alcohol and be cautious with activities requiring alertness. Grapefruit juice does not significantly affect pethidine pharmacokinetics.
Clinical pearls
Exam focus: The most high-yield testable concept is that pethidine produces norpethidine, a neurotoxic metabolite that accumulates in renal failure and causes seizures that are not reversed by naloxone. This differentiates it from other opioids.
Declining use: Pethidine use has fallen dramatically in modern practice. It offers no advantages over other opioids and has unique toxicity risks. Many hospitals have removed it from formularies. When a question stem describes an opioid causing seizures in a patient with renal disease, think pethidine.
Anticholinergic effects: Unlike most opioids, pethidine causes tachycardia rather than bradycardia due to its atropine-like effects. It also causes less miosis than morphine.
MAOI interaction: The pethidine-MAOI interaction is one of the most dangerous drug interactions in medicine. Even a single dose of pethidine in a patient taking MAOIs can be fatal. Always screen for MAOI use.
Duration of action: Pethidine has a relatively short duration (2-4 hours), requiring frequent dosing. This contributes to norpethidine accumulation with repeated administration.
Not for chronic pain: Never use pethidine for chronic pain management. Limit use to 48-72 hours maximum to minimize norpethidine accumulation.
Rigors: One of the few remaining indications is treatment of drug-induced rigors (amphotericin B) or postoperative shivering, where a single dose may be effective.
Histamine release: Like morphine, pethidine can cause histamine release leading to pruritus, flushing, and hypotension. This is not a true allergy.
Sickle cell crisis: Historically used in sickle cell pain crises, but now largely replaced by morphine or hydromorphone due to seizure risk, especially in patients with renal complications.
Comparison with morphine: Pethidine has 1/10th the potency, shorter duration, more tachycardia, less GI spasm, and unique neurotoxicity via norpethidine. Morphine is generally preferred.
PETHIDINE problems: Pethidine → Excitatory metabolite (norpethidine) → Tremors/Twitching/Tachycardia → Harms kidneys (accumulates) → Induces seizures → Dangerous with MAOIs → In renal failure avoid → Naloxone doesn't help norpethidine → Elderly at risk
Practice questions
1. A 68-year-old man with chronic kidney disease (creatinine 3.2 mg/dL) undergoes hip replacement surgery. He receives pethidine intramuscularly every 4 hours for postoperative pain. On day 3, he develops tremors, myoclonic jerks, and a generalized tonic-clonic seizure. Which of the following best explains this presentation?
2. A 45-year-old woman with depression treated with phenelzine presents to the emergency department with severe acute pain from a kidney stone. The physician considers giving pethidine for analgesia. Which of the following is the most appropriate next step?
3. A 55-year-old man develops severe shivering and rigors 30 minutes after starting an amphotericin B infusion for invasive aspergillosis. His temperature is 39.2°C (102.6°F), blood pressure is 145/90 mmHg, and pulse is 110/min. He has normal renal function. Which of the following medications is most appropriate for immediate management of his rigors?
Frequently asked questions
How does pethidine (meperidine) work?
Pethidine works by binding to μ-opioid receptors in the brain and spinal cord, blocking pain signals and altering pain perception. Unlike other opioids, it also has anticholinergic properties that cause side effects like tachycardia and dry mouth. It is metabolized to norpethidine, a toxic metabolite that can accumulate with repeated doses or in kidney disease, causing tremors and seizures that are not reversed by naloxone.
What are the side effects of pethidine?
Common side effects include sedation, dizziness, nausea, vomiting, constipation, dry mouth, and tachycardia (faster heart rate). The most serious unique side effect is neurotoxicity from norpethidine accumulation, causing tremors, muscle twitching, and seizures, especially in patients with kidney disease or after repeated doses. Other serious effects include respiratory depression, severe hypotension, and potentially fatal serotonin syndrome when combined with certain antidepressants or MAOIs.
Why is pethidine not used anymore?
Pethidine use has declined dramatically because it offers no advantages over other opioids like morphine or fentanyl, while having unique and serious toxicity risks. Its metabolite norpethidine accumulates with repeated use and in kidney disease, causing seizures that naloxone cannot reverse. The dangerous interaction with MAOIs and serotonergic drugs, combined with its short duration requiring frequent dosing, has led many hospitals to remove it from their medication formularies entirely.
Can pethidine cause seizures?
Yes, pethidine can cause seizures through its metabolite norpethidine, which accumulates in the body, especially with repeated doses or in patients with kidney disease. Norpethidine is a CNS stimulant that lowers the seizure threshold and can cause tremors, muscle twitching, hyperreflexia, and generalized tonic-clonic seizures. Importantly, naloxone does not reverse these seizures and may even worsen them. This unique toxicity is a major reason pethidine is no longer recommended for routine pain management.
Pethidine vs morphine — what is the difference?
Pethidine has about 1/10th the potency of morphine and a shorter duration of action (2-4 hours vs 4-6 hours). Unlike morphine, pethidine has anticholinergic effects causing tachycardia rather than bradycardia. The key difference is that pethidine produces norpethidine, a neurotoxic metabolite that can cause seizures, especially in renal impairment—a problem morphine does not have. Pethidine also has a dangerous interaction with MAOIs. For these reasons, morphine is generally preferred for most clinical situations.
Conditions it is used for
Related drugs
Sources and further reading
DailyMed (FDA labels)StatPearls (NCBI Bookshelf)MedlinePlusWHO ATC/DDD index: N02AB02
Educational content for medical students, prepared by the WhiteCoat Pro editorial team with AI assistance; every page is reviewed by a second AI model for factual errors. It is not medical advice and does not replace the product label, local guidelines or a clinician.