Head-to-head clinical analysis & difference comparison: details on mechanism of action, dosing, half-life, interactions, and maternal-fetal safety.
NUMORPHAN vs ACEPHEN
Clinician-reviewed, head-to-head comparison of mechanism, dosing, pharmacokinetics, and safety profiles.
Last clinically reviewed: July 2026 · OpiCalc Medical Review Team
Opioid agonist; binds to mu-opioid receptors in the CNS, inhibiting ascending pain pathways and altering pain perception.
ACEPHEN (acetaminophen) is a para-aminophenol derivative with analgesic and antipyretic activity. Its mechanism involves inhibition of cyclooxygenase (COX) enzymes in the central nervous system, particularly COX-2, reducing prostaglandin synthesis. It has weak peripheral COX inhibition and minimal anti-inflammatory effect.
Moderate to severe pain where an opioid analgesic is appropriate,Anesthesia adjunct (off-label)
Mild to moderate pain,Fever
Intravenous or subcutaneous: 0.5-2 mg (0.1-0.2 mg/kg for severe pain) every 2-3 hours as needed; not to exceed 20 mg/day.
325-650 mg orally every 4-6 hours as needed; maximum 4 g/day.
Terminal elimination half-life is 2–3 hours in adults; prolonged to 3–4 hours in elderly and up to 15 hours in patients with severe hepatic impairment.
Terminal elimination half-life: 1.0-1.5 hours in adults with normal renal function. Prolonged to 2-5 hours in hepatic impairment or elderly; requires dose adjustment in severe hepatic disease.
Primarily hepatic via glucuronidation; involves UGT2B7; active metabolite (hydromorphone-3-glucuronide) may accumulate in renal impairment.
Acetaminophen is primarily metabolized in the liver via glucuronidation (UGT1A1, UGT1A6, UGT1A9) and sulfation (SULT1A1, SULT1A3). A minor fraction is oxidized by cytochrome P450 enzymes (CYP2E1, CYP1A2, CYP3A4) to a reactive toxic metabolite (NAPQI), which is normally detoxified by conjugation with glutathione.
Primarily renal (approximately 70% as unchanged drug, <5% as noroxymorphone and other conjugates); biliary/fecal excretion accounts for ~20%.
Renal: 90-95% as unchanged drug; tubular secretion and glomerular filtration. Biliary/fecal: <5%.
Approximately 10–20% bound to plasma proteins (primarily albumin).
Approximately 10-20% bound to serum albumin; extensive tissue binding.
2.5–3.5 L/kg; indicates extensive tissue distribution, with high affinity for brain and other tissues.
Apparent Vd: 0.5-0.7 L/kg (30-40 L in a 70 kg adult). Distributions into CSF and breast milk.
Intravenous: 100%; Intramuscular: approximately 85–95%; Subcutaneous: approximately 75–85%; Oral: approximately 10–20% due to extensive first-pass metabolism.
Oral: 85-90% (first-pass metabolism minimal). Rectal: approximately 70-80% of oral bioavailability.
Cr Cl 10-50 m L/min: administer 75% of dose; Cr Cl <10 m L/min: administer 50% of dose, use with caution, consider extended dosing intervals.
GFR 10-50 m L/min: 650 mg every 6 hours; GFR <10 m L/min: 650 mg every 8 hours.
Child-Pugh Class B: reduce dose by 50%; Child-Pugh Class C: reduce dose by 75% or avoid use; start at lowest dose and titrate carefully.
Child-Pugh Class A: no adjustment; Child-Pugh Class B: maximum 2 g/day; Child-Pugh Class C: maximum 1 g/day.
Children >2 years: 0.1-0.2 mg/kg intravenously, subcutaneously, or intramuscularly every 2-4 hours as needed; not to exceed 15 mg/day.
10-15 mg/kg/dose orally every 4-6 hours; maximum 75 mg/kg/day or 4 g/day, whichever is less.
Start at low end of dosing range (0.5 mg), extend dosing interval to 3-4 hours, monitor for respiratory depression and constipation, use non-opioid alternatives when possible.
Start at lowest effective dose (325 mg every 6 hours); avoid exceeding 3 g/day unless closely monitored.
Risk of respiratory depression, especially in elderly, cachectic, or debilitated patients; concomitant use with benzodiazepines or CNS depressants may cause profound sedation, respiratory depression, coma, and death; abuse potential leading to addiction; neonatal opioid withdrawal syndrome with prolonged use during pregnancy.
Acetaminophen has been associated with cases of acute liver failure, at times resulting in liver transplant and death. Most of the cases of liver injury are associated with the use of acetaminophen at doses that exceed 4,000 milligrams per day, and often involve more than one acetaminophen-containing product.
Respiratory depression; CNS depression; serotonin syndrome (with serotonergic drugs); adrenal insufficiency; hypotension; seizures; opioid-induced hyperalgesia; impaired ability to drive/operate machinery; risk of overdose with alcohol or other CNS depressants.
Risk of severe liver injury with doses >4000 mg/day; use caution with hepatic impairment, chronic alcoholism, malnutrition, or concomitant hepatotoxic drugs; avoid exceeding recommended dose; limit use to 10 days for pain or 3 days for fever unless directed by physician; serious skin reactions (Stevens-Johnson syndrome, toxic epidermal necrolysis) have occurred.
Hypersensitivity to hydromorphone or any component; significant respiratory depression; acute or severe bronchial asthma; known or suspected paralytic ileus; concurrent use of MAOIs (or within 14 days).
Hypersensitivity to acetaminophen or any component of the formulation; severe hepatic impairment or active liver disease.
Avoid alcohol and grapefruit juice. Grapefruit juice may inhibit CYP3A4 metabolism of oxymorphone, leading to increased plasma concentrations and risk of toxicity. High-fat meals may delay absorption but not significantly alter overall exposure. Maintain adequate hydration to prevent constipation.
Alcohol: increased risk of hepatotoxicity. Avoid concurrent use. Food: no significant interaction, but taking with food may reduce minor gastrointestinal irritation.
First trimester: Limited data, but opioid use is associated with neural tube defects and congenital heart defects in some studies. Second and third trimesters: Chronic use may lead to fetal opioid dependence and neonatal withdrawal syndrome. No specific malformation pattern attributed to Numorphan. Avoid prolonged use, especially in first trimester.
Pregnancy Category C. First trimester: potential risk of neural tube defects and orofacial clefts (limited human data, animal studies show embryotoxicity). Second and third trimesters: NSAID exposure associated with oligohydramnios, premature ductus arteriosus constriction, and fetal renal impairment. Avoid in third trimester.
Numorphan (oxymorphone) is excreted into breast milk. The M/P ratio is unknown. Limited data suggest low concentrations, but caution is advised. Monitor infant for drowsiness, respiratory depression, and withdrawal if maternal use is prolonged. American Academy of Pediatrics considers it compatible with breastfeeding with cautious use, but consider risk-benefit.
Excreted into breast milk in low concentrations (M/P ratio approximately 0.10). Considered compatible with breastfeeding; however, use lowest effective dose for shortest duration given potential for neonatal adverse effects (e.g., thrombocytopenia, renal dysfunction).
Pregnancy increases renal blood flow and hepatic metabolism; clearance of oxymorphone may be increased. No formal studies available. Consider dose adjustments based on clinical response and tolerability. Avoid in first trimester if possible; if necessary, use lowest effective dose for shortest duration. In third trimester, anticipate potential neonatal withdrawal and plan for taper postpartum.
No standard dose adjustments recommended; however, due to increased plasma volume and metabolism in pregnancy, higher doses may be required to achieve therapeutic effect. Avoid near term.
Numorphan (oxymorphone) is a potent semisynthetic opioid agonist approximately 10 times more potent than morphine. Avoid in patients with severe respiratory depression, paralytic ileus, or acute asthma. Caution in elderly, cachectic, or debilitated patients due to increased risk of respiratory depression. Monitor for hypotension and bradycardia. Use with extreme caution in patients with head injury or increased intracranial pressure. Abrupt discontinuation may precipitate withdrawal; taper gradually.
ACEPHEN (acetaminophen) is commonly used for mild to moderate pain and fever. Avoid exceeding 4 g/day in adults to prevent hepatotoxicity. In patients with hepatic impairment, reduce maximum daily dose to 2 g. Consider acetylcysteine for overdose. Onset of action is 15-30 minutes orally.
Take exactly as prescribed; do not increase dose or frequency without consulting your doctor.,Numorphan may cause dizziness, drowsiness, or blurred vision; avoid driving or operating heavy machinery until you know how the drug affects you.,Avoid alcohol and other central nervous system depressants (e.g., sedatives, tranquilizers) as they may increase the risk of serious side effects like respiratory depression.,If you miss a dose, skip it and take the next dose at the scheduled time; do not double up.,Do not suddenly stop taking this medication; a gradual reduction under medical supervision is needed to prevent withdrawal symptoms.,Store securely away from children and pets; dispose of unused medication via a drug take-back program.
Do not exceed 4000 mg (4 grams) in 24 hours.,Avoid drinking alcohol while taking this medication.,Do not combine with other products containing acetaminophen.,Take with food if stomach upset occurs.,Seek immediate medical help if you experience symptoms of liver damage: yellowing of skin/eyes, dark urine, severe abdominal pain.
No interactions on record
No interactions on record
Explore head-to-head clinical comparisons of other medications in the same therapeutic classes.
Common clinical questions about NUMORPHAN vs ACEPHEN, answered by our medical review team.
NUMORPHAN is a Opioid Analgesic that works by Opioid agonist; binds to mu-opioid receptors in the CNS, inhibiting ascending pain pathways and altering pain perception.. ACEPHEN is a Non-Opioid Analgesic that works by ACEPHEN (acetaminophen) is a para-aminophenol derivative with analgesic and antipyretic activity. Its mechanism involves inhibition of cyclooxygenase (COX) enzymes in the central nervous system, particularly COX-2, reducing prostaglandin synthesis. It has weak peripheral COX inhibition and minimal anti-inflammatory effect.. They differ in pharmacokinetic profiles, FDA-approved indications, and side effect profiles.
Potency comparisons between NUMORPHAN and ACEPHEN depend on the specific clinical indication. These are agents from distinct pharmacological classes and are not directly interchangeable by dose. A physician or clinical pharmacist should guide any therapeutic switching decisions.
The standard adult dose of NUMORPHAN is: Intravenous or subcutaneous: 0.5-2 mg (0.1-0.2 mg/kg for severe pain) every 2-3 hours as needed; not to exceed 20 mg/day.. The standard adult dose of ACEPHEN is: 325-650 mg orally every 4-6 hours as needed; maximum 4 g/day.. Dosing should always be individualized based on indication, renal and hepatic function, age, and other patient factors.
No direct drug-drug interaction has been formally documented between NUMORPHAN and ACEPHEN in current clinical databases. However, individual patient risk factors including other medications, organ function, and comorbidities should always be evaluated by a qualified healthcare provider.
The maternal-fetal safety profiles differ. NUMORPHAN is classified as Category C. First trimester: Limited data, but opioid use is associated with neural tube defects and congenital heart defects in some studies. Second and third trimesters: Chronic use may lead. ACEPHEN is classified as Category C. Pregnancy Category C. First trimester: potential risk of neural tube defects and orofacial clefts (limited human data, animal studies show embryotoxicity). Second and third trimest. Always consult a maternal-fetal medicine specialist before taking either drug during pregnancy or lactation.