Head-to-head clinical analysis & difference comparison: details on mechanism of action, dosing, half-life, interactions, and maternal-fetal safety.
ACEPHEN vs ACTAHIST
Clinician-reviewed, head-to-head comparison of mechanism, dosing, pharmacokinetics, and safety profiles.
Last clinically reviewed: July 2026 · OpiCalc Medical Review Team
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.
Antihistamine; binds to histamine H1 receptors, blocking the effects of histamine; also exhibits anticholinergic and mild sedative properties.
Mild to moderate pain,Fever
Symptomatic relief of allergic rhinitis,Urticaria,Off-label: motion sickness,Off-label: insomnia
325-650 mg orally every 4-6 hours as needed; maximum 4 g/day.
1.34 mg (one capsule) orally twice daily.
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.
6.9 ± 1.7 hours in adults; prolonged to 12-18 hours in elderly or patients with hepatic impairment, requiring dosing interval adjustment.
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.
Hepatic metabolism via CYP450 enzymes (primarily CYP3A4 and CYP2D6); major metabolite is inactive.
Renal: 90-95% as unchanged drug; tubular secretion and glomerular filtration. Biliary/fecal: <5%.
Primarily renal (approximately 85% as unchanged drug and metabolites) and fecal (15%) via biliary elimination.
Approximately 10-20% bound to serum albumin; extensive tissue binding.
92% bound to albumin.
Apparent Vd: 0.5-0.7 L/kg (30-40 L in a 70 kg adult). Distributions into CSF and breast milk.
0.9 ± 0.3 L/kg, indicating extensive extravascular distribution.
Oral: 85-90% (first-pass metabolism minimal). Rectal: approximately 70-80% of oral bioavailability.
Oral: 68% ± 12% due to first-pass metabolism.
GFR 10-50 m L/min: 650 mg every 6 hours; GFR <10 m L/min: 650 mg every 8 hours.
No dose adjustment required for mild to moderate renal impairment. Safety not established for severe impairment (GFR <30 m L/min).
Child-Pugh Class A: no adjustment; Child-Pugh Class B: maximum 2 g/day; Child-Pugh Class C: maximum 1 g/day.
No dose adjustment required for mild to moderate hepatic impairment (Child-Pugh A or B). Not recommended for severe hepatic impairment (Child-Pugh C).
10-15 mg/kg/dose orally every 4-6 hours; maximum 75 mg/kg/day or 4 g/day, whichever is less.
Not indicated for pediatric patients under 12 years of age. Safety and efficacy not established.
Start at lowest effective dose (325 mg every 6 hours); avoid exceeding 3 g/day unless closely monitored.
No specific dose adjustment recommended; monitor for increased anticholinergic effects and cognitive impairment.
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.
None.
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.
May cause drowsiness; caution when driving or operating machinery. Avoid alcohol. Use with caution in patients with narrow-angle glaucoma, prostatic hyperplasia, or urinary retention. Geriatric patients more sensitive to anticholinergic effects. Pediatric patients <6 years: not recommended.
Hypersensitivity to acetaminophen or any component of the formulation; severe hepatic impairment or active liver disease.
Hypersensitivity to any component. Newborns or premature infants. Breastfeeding (contraindicated due to risk of adverse effects in infants). Concomitant use with MAOIs.
Alcohol: increased risk of hepatotoxicity. Avoid concurrent use. Food: no significant interaction, but taking with food may reduce minor gastrointestinal irritation.
Avoid high-tyramine foods (aged cheese, cured meats, fermented products) if taking MAOIs. Grapefruit juice may increase phenylephrine absorption; limit intake.
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.
ACTAHIST (brompheniramine/phenylephrine) pregnancy category C. Inadequate human data; animal studies show no malformations at therapeutic doses. First trimester: theoretical risk from vasoconstrictive effects (phenylephrine) possibly reducing uterine blood flow; avoid if possible. Second/third trimester: phenylephrine may cause fetal hypoxia via placental vasoconstriction; use only if benefit outweighs risk. No known structural teratogenicity.
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).
Brompheniramine is excreted in breast milk in small amounts; M/P ratio not established. Phenylephrine has minimal excretion. Due to anticholinergic effects, may reduce milk production or cause sedation in infants. Use caution; prefer non-sedating alternatives if possible.
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.
No specific pharmacokinetic studies. Increased plasma volume and renal clearance in pregnancy may reduce drug levels, but efficacy threshold remains. No dose adjustment recommended; use the lowest effective dose for shortest duration due to potential risks.
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.
Actahist is a combination antihistamine-decongestant (chlorpheniramine/phenylephrine). Avoid in patients with hypertension, severe coronary artery disease, or MAOI use. Monitor for sedation and urinary retention, especially in elderly males with BPH.
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.
Take with food or milk to reduce stomach upset.,Avoid alcohol and CNS depressants as they can increase drowsiness.,Do not drive or operate machinery until you know how this medication affects you.,Contact your doctor if you experience chest pain, rapid heartbeat, or difficulty urinating.
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 ACEPHEN vs ACTAHIST, answered by our medical review team.
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.. ACTAHIST is a Antihistamine that works by Antihistamine; binds to histamine H1 receptors, blocking the effects of histamine; also exhibits anticholinergic and mild sedative properties.. They differ in pharmacokinetic profiles, FDA-approved indications, and side effect profiles.
Potency comparisons between ACEPHEN and ACTAHIST 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 ACEPHEN is: 325-650 mg orally every 4-6 hours as needed; maximum 4 g/day.. The standard adult dose of ACTAHIST is: 1.34 mg (one capsule) orally twice daily.. 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 ACEPHEN and ACTAHIST 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. 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. ACTAHIST is classified as Category C. ACTAHIST (brompheniramine/phenylephrine) pregnancy category C. Inadequate human data; animal studies show no malformations at therapeutic doses. First trimester: theoretical risk f. Always consult a maternal-fetal medicine specialist before taking either drug during pregnancy or lactation.