Head-to-head clinical analysis & difference comparison: details on mechanism of action, dosing, half-life, interactions, and maternal-fetal safety.
DARVON-N vs ACEPHEN
Clinician-reviewed, head-to-head comparison of mechanism, dosing, pharmacokinetics, and safety profiles.
Last clinically reviewed: July 2026 · OpiCalc Medical Review Team
Propoxyphene is a weak mu-opioid receptor agonist that produces analgesia by binding to opioid receptors in the central nervous system, altering the perception of and response to pain. Its metabolite norpropoxyphene has local anesthetic and sodium channel blocking effects, which may contribute to cardiac toxicity.
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.
Mild to moderate pain,Off-label: None established
Mild to moderate pain,Fever
100 mg orally every 4 hours as needed for pain; maximum 600 mg per day.
325-650 mg orally every 4-6 hours as needed; maximum 4 g/day.
Propoxyphene: 6-12 hours; norpropoxyphene: 30-36 hours. Accumulation of norpropoxyphene on repeated dosing increases risk of toxicity.
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 CYP3A4 and CYP2D6 to norpropoxyphene (active metabolite). Propoxyphene and norpropoxyphene undergo further metabolism and conjugation.
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 and glucuronide conjugates); minor biliary/fecal elimination (25-30%).
Renal: 90-95% as unchanged drug; tubular secretion and glomerular filtration. Biliary/fecal: <5%.
Propoxyphene: 70-80%; norpropoxyphene: 75-85%. Primarily bound to albumin.
Approximately 10-20% bound to serum albumin; extensive tissue binding.
Propoxyphene: 12-26 L/kg; widely distributed into tissues, including CNS.
Apparent Vd: 0.5-0.7 L/kg (30-40 L in a 70 kg adult). Distributions into CSF and breast milk.
Oral: approximately 30-70% due to extensive first-pass metabolism; interindividual variability.
Oral: 85-90% (first-pass metabolism minimal). Rectal: approximately 70-80% of oral bioavailability.
GFR 30-89 m L/min: no adjustment; GFR <30 m L/min: reduce dose by 50% or extend interval to every 8-12 hours; not recommended for GFR <15 m L/min.
GFR 10-50 m L/min: 650 mg every 6 hours; GFR <10 m L/min: 650 mg every 8 hours.
Child-Pugh Class A: no adjustment; Child-Pugh Class B: reduce dose by 50%; Child-Pugh Class C: avoid use.
Child-Pugh Class A: no adjustment; Child-Pugh Class B: maximum 2 g/day; Child-Pugh Class C: maximum 1 g/day.
Not recommended for children under 12 years; for adolescents 12-17 years: 100 mg every 4 hours as needed, maximum 400 mg per day.
10-15 mg/kg/dose orally every 4-6 hours; maximum 75 mg/kg/day or 4 g/day, whichever is less.
Initiate at 50 mg every 4 hours as needed; maximum 400 mg per day; monitor for CNS depression and constipation.
Start at lowest effective dose (325 mg every 6 hours); avoid exceeding 3 g/day unless closely monitored.
DARVON-N (propoxyphene) is contraindicated in patients with a history of drug abuse, and its use should be monitored for signs of abuse, addiction, or misuse. It should not be used in patients with severe respiratory depression, acute or severe bronchial asthma, or known hypersensitivity to propoxyphene. Additionally, because of the risk of QT prolongation and cardiac arrhythmias, propoxyphene-containing products were withdrawn from the US market in 2010.
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.
Addiction, abuse, and misuse,Respiratory depression,Accidental ingestion (especially in children) can be fatal,Neonatal opioid withdrawal syndrome,Interactions with CNS depressants (e.g., alcohol, benzodiazepines),Elderly or debilitated patients: increased risk of respiratory depression,Hepatic or renal impairment,QT prolongation and risk of torsade de pointes
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 propoxyphene or any component,Significant respiratory depression,Acute or severe bronchial asthma in an unmonitored setting,Known or suspected gastrointestinal obstruction (e.g., paralytic ileus),Concurrent use of alcohol, sedatives, or other CNS depressants,History of substance abuse,Concurrent MAOIs or within 14 days
Hypersensitivity to acetaminophen or any component of the formulation; severe hepatic impairment or active liver disease.
Avoid alcohol; may enhance CNS depression. No specific food interactions known, but high-fat meals may delay absorption.
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; potential for neural tube defects with first-trimester exposure. Second and third trimesters: Risk of premature closure of ductus arteriosus, oligohydramnios, and neonatal withdrawal syndrome. Use only if clearly needed.
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.
Propoxyphene is excreted into breast milk with a milk-to-plasma ratio of approximately 1.0. Theoretical risk of neonatal respiratory depression and withdrawal. Not recommended; consider alternative analgesics.
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).
Increased clearance in pregnancy may necessitate higher doses; however, due to risks, avoid use or limit to lowest effective dose for shortest duration. No established dose adjustment guidelines.
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.
Darvon-N (propoxyphene napsylate) is a weak opioid analgesic with efficacy similar to codeine; its use is limited by narrow therapeutic index and risk of QT prolongation. It is metabolized by CYP2D6 to norpropoxyphene, which has a long half-life and can accumulate, causing CNS toxicity. Due to safety concerns, it has been withdrawn from many markets; avoid in patients with substance use disorder, renal impairment, or those taking CNS depressants. Monitor for QTc prolongation if used with other QT-prolonging agents.
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 due to risk of serious side effects including seizures and cardiac arrhythmias.,Avoid alcohol and other CNS depressants (e.g., benzodiazepines, sedatives) as they increase risk of respiratory depression and sedation.,Do not stop abruptly after prolonged use; withdrawal symptoms may occur.,Store securely out of reach of children; dispose of unused medication properly.,Use caution when driving or operating machinery; may cause dizziness or drowsiness.,Report new or worsening shortness of breath, irregular heartbeat, or fainting to your healthcare provider.
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 DARVON-N vs ACEPHEN, answered by our medical review team.
DARVON-N is a Opioid Analgesic that works by Propoxyphene is a weak mu-opioid receptor agonist that produces analgesia by binding to opioid receptors in the central nervous system, altering the perception of and response to pain. Its metabolite norpropoxyphene has local anesthetic and sodium channel blocking effects, which may contribute to cardiac toxicity.. 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 DARVON-N 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 DARVON-N is: 100 mg orally every 4 hours as needed for pain; maximum 600 mg per 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 DARVON-N 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. DARVON-N is classified as Category C. First trimester: Limited data; potential for neural tube defects with first-trimester exposure. Second and third trimesters: Risk of premature closure of ductus arteriosus, oligohy. 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.