Head-to-head clinical analysis & difference comparison: details on mechanism of action, dosing, half-life, interactions, and maternal-fetal safety.
ACYCLOVIR SODIUM vs OFIRMEV
Clinician-reviewed, head-to-head comparison of mechanism, dosing, pharmacokinetics, and safety profiles.
Last clinically reviewed: July 2026 · OpiCalc Medical Review Team
Acyclovir is a synthetic nucleoside analogue with activity against herpes simplex virus (HSV) types 1 and 2, and varicella-zoster virus (VZV). It is converted to acyclovir monophosphate by viral thymidine kinase, then further phosphorylated to acyclovir triphosphate, which competitively inhibits viral DNA polymerase and incorporates into viral DNA, causing chain termination.
OFIRMEV (acetaminophen) is a para-aminophenol derivative with analgesic and antipyretic activity. Its mechanism of action is not fully understood, but it is thought to involve inhibition of cyclooxygenase (COX) enzymes in the central nervous system, with minimal peripheral COX inhibition. It may also act on serotonergic pathways and cannabinoid receptors.
Treatment of initial and recurrent genital herpes in immunocompetent patients,Treatment of herpes simplex encephalitis,Treatment of neonatal herpes simplex virus infection,Treatment of varicella-zoster (shingles) in immunocompetent and immunocompromised patients,Treatment of mucocutaneous herpes simplex in immunocompromised patients,Prophylaxis of herpes simplex in immunocompromised patients (off-label)
Management of mild to moderate pain,Management of moderate to severe pain with adjunctive opioid analgesics,Reduction of fever
Dosing is indication-specific. For herpes simplex encephalitis: 10 mg/kg IV every 8 hours for 10–14 days (adults and children ≥12 years) or 20 mg/kg IV every 8 hours (3 months–12 years). For severe genital herpes: 5 mg/kg IV every 8 hours for 5 days. For mucocutaneous HSV in immunocompromised: 5 mg/kg IV every 8 hours for 7–14 days. For varicella zoster in immunocompromised: 10 mg/kg IV every 8 hours for 7 days. For neonatal HSV: 20 mg/kg IV every 8 hours for 14–21 days (disseminated/CNS) or 14 days (skin/eyes/mouth).
IV: 1000 mg every 6 hours or 650 mg every 4 hours; maximum single dose: 1000 mg; minimum dosing interval: 4 hours; maximum daily dose: 4000 mg.
Terminal elimination half-life: 2.5-3.3 hours in adults with normal renal function; up to 20 hours in anuria/end-stage renal disease.
Terminal elimination half-life is 2-3 hours in adults (2.5-3 hours in children). Clinically, dosing every 4-6 hours is needed to maintain therapeutic levels.
Acyclovir is primarily excreted unchanged in the urine via glomerular filtration and tubular secretion. Hepatic metabolism is minimal, with less than 15% metabolized to 9-carboxymethoxymethylguanine via alcohol dehydrogenase and aldehyde dehydrogenase.
Acetaminophen is primarily metabolized in the liver via conjugation with glucuronide (50-60%) and sulfate (20-30%). A minor amount is oxidized by cytochrome P450 (CYP2E1, CYP1A2, CYP3A4) to a toxic reactive metabolite (NAPQI), which is normally detoxified by glutathione. At toxic doses, glutathione is depleted, leading to NAPQI accumulation and hepatotoxicity.
Primarily renal excretion via glomerular filtration and tubular secretion: 62-91% of dose excreted unchanged in urine within 24 hours; minor biliary/fecal elimination (<2%).
Primarily renal (85% as sulfate and glucuronide conjugates, 10% as unchanged drug). Less than 5% fecal/biliary.
9-33% bound primarily to albumin.
10-25% bound to albumin at therapeutic concentrations.
0.6-1.0 L/kg; approximates total body water, indicating wide distribution including into vesicles and CSF (CSF concentrations ~50% of plasma).
0.8-1.0 L/kg. Indicates distribution into total body water.
Oral: 10-20% (dose-dependent, saturable absorption); topical: negligible systemic absorption.
100% (intravenous); not applicable for other routes as OFIRMEV is IV only.
Adjust dosing interval based on creatinine clearance (Cr Cl): Cr Cl >50 m L/min: standard dose every 8 hours. Cr Cl 25–50 m L/min: standard dose every 12 hours. Cr Cl 10–25 m L/min: standard dose every 24 hours. Cr Cl 0–10 m L/min: reduce dose by 50% and administer every 24 hours. Hemodialysis: administer after dialysis; typically 50% of standard dose every 24 hours, with a supplemental dose post-dialysis.
No dose adjustment required for GFR ≥30 m L/min. For GFR <30 m L/min, extend dosing interval to every 8 hours; maximum daily dose 3000 mg.
No dosage adjustment required in isolated hepatic impairment; caution if concomitant renal dysfunction.
Child-Pugh Class A: No adjustment. Child-Pugh Class B: Reduce total daily dose by 50% (max 2000 mg/day). Child-Pugh Class C: Contraindicated or use with extreme caution; reduce dose to 50% of standard and extend interval to every 8 hours; maximum 2000 mg/day.
Indicated in neonates and children. Neonates: 20 mg/kg/dose IV every 8 hours. Infants >3 months: 10–20 mg/kg/dose every 8 hours based on indication. For HSV encephalitis: children 3 months–12 years: 20 mg/kg/dose every 8 hours; ≥12 years: 10 mg/kg/dose every 8 hours. Doses are based on ideal body weight in obese patients.
Weight-based: <10 kg: 7.5 mg/kg/dose every 6 hours; 10-50 kg: 15 mg/kg/dose every 6 hours; >50 kg: 1000 mg every 6 hours or 650 mg every 4 hours. Maximum single dose: 15 mg/kg (up to 1000 mg); maximum daily dose: 75 mg/kg (up to 4000 mg).
No age-specific dose adjustment; dose adjustments are based on renal function, which is often reduced in the elderly. Monitor renal function closely and consider risk of neurotoxic side effects.
No specific dose adjustment; consider reduced renal function. For Cr Cl <30 m L/min, extend interval to every 8 hours. Maximum daily dose: 3000 mg in frail elderly or with comorbidities.
None.
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 4000 mg per day, and often involve more than one acetaminophen-containing product.
Renal impairment: Dose adjustment required in patients with decreased renal function.,Neurotoxicity: May cause tremors, seizures, hallucinations, or confusion, particularly in elderly patients or those with renal impairment.,Hydration: Ensure adequate hydration during administration to prevent renal tubule crystallization.,Thrombotic thrombocytopenic purpura/hemolytic uremic syndrome (TTP/HUS) reported in immunocompromised patients.,Do not administer by intramuscular or subcutaneous injection due to tissue irritation.
Risk of serious hepatotoxicity, especially with doses >4000 mg/day or in patients with underlying liver disease,Risk of severe skin reactions (Stevens-Johnson syndrome, toxic epidermal necrolysis, acute generalized exanthematous pustulosis) – discontinue at first sign of rash,Risk of hypersensitivity reactions including anaphylaxis,Use caution in patients with severe hepatic impairment, active hepatic disease, or alcoholism,Avoid concurrent use of other acetaminophen-containing products
Hypersensitivity to acyclovir or valacyclovir
Known hypersensitivity to acetaminophen or any component of the formulation,Severe hepatic impairment or active liver disease (relative contraindication without black box)
No significant food interactions. Maintain adequate fluid intake to prevent renal precipitation.
No known food interactions. However, avoid excessive alcohol consumption as it may increase the risk of liver damage.
Pregnancy Category B. No evidence of teratogenicity in humans; fetal risks not established in first trimester. Use during pregnancy only if clearly needed.
Acetaminophen (OFIRMEV) is generally considered low risk across all trimesters. No increased risk of major congenital anomalies has been consistently demonstrated. Chronic high-dose use in third trimester may be associated with preterm birth or low birth weight. Avoid prolonged use above recommended doses.
Acyclovir is excreted in breast milk; M/P ratio 0.6-4.1. Typically compatible with breastfeeding; monitor infant for rash or gastrointestinal disturbances.
Acetaminophen is excreted in breast milk in low concentrations (M/P ratio approximately 0.9-1.0). Considered compatible with breastfeeding; peak milk levels occur 1-2 hours after maternal dosing. Use lowest effective dose for shortest duration.
No routine dose adjustment; pharmacokinetic changes in pregnancy may require increased dosing due to increased clearance and volume of distribution, especially in third trimester. Monitor clinical response.
No dose adjustment required during pregnancy. Pharmacokinetic changes in pregnancy (increased volume of distribution, clearance) may lead to lower peak concentrations but standard dosing remains effective. Maximum single dose: 1 g; maximum daily dose: 4 g.
Monitor renal function closely; adjust dose in renal impairment. Ensure adequate hydration to prevent crystalluria. Infuse over at least 1 hour to avoid phlebitis. Use with caution in elderly and those with pre-existing renal disease. Neurotoxicity may occur at high doses or in renal failure. Not effective for EBV or CMV treatment.
OFIRMEV (acetaminophen) injection is an IV formulation of acetaminophen used for pain and fever management. It is a prodrug that requires no hepatic conversion, providing rapid onset of action. Monitor for hepatotoxicity; maximum daily dose is 4 grams in adults but lower in patients with hepatic impairment or malnutrition. Do not exceed 1 gram per dose. Hypotension and anaphylaxis have been reported. Not interchangeable with oral acetaminophen due to dose equivalency. Use with caution in patients with alcohol use disorder.
Drink plenty of water during treatment to prevent kidney problems.,Report any signs of kidney issues like decreased urine output or swelling.,Notify healthcare provider if you experience confusion, hallucinations, or seizures.,This medication is for intravenous use only and will be given in a medical setting.,Inform your doctor about all medications you are taking, especially other nephrotoxic drugs.
OFIRMEV is given intravenously for pain or fever.,Do not take additional acetaminophen-containing medications while receiving OFIRMEV.,Report any signs of allergic reaction (rash, itching, swelling, trouble breathing).,Seek immediate medical attention if you experience severe abdominal pain, yellowing of skin or eyes, or dark urine.,Inform your healthcare provider about all medications you are taking, especially blood thinners.
"Teriflunomide, the active metabolite of leflunomide, inhibits dihydroorotate dehydrogenase (DHODH), a key enzyme in de novo pyrimidine synthesis, exerting immunomodulatory effects. Acyclovir, an antiviral nucleoside analog, may inhibit organic anion transporter 3 (OAT3)-mediated renal tubular secretion of teriflunomide, leading to increased systemic exposure. Elevated teriflunomide concentrations can potentiate hepatotoxicity, myelosuppression, and immunosuppression, increasing the risk of infections and other adverse effects."
"The serum concentration of Acyclovir can be increased when it is combined with Tizanidine."
No interactions on record
Explore head-to-head clinical comparisons of other medications in the same therapeutic classes.
Common clinical questions about ACYCLOVIR SODIUM vs OFIRMEV, answered by our medical review team.
ACYCLOVIR SODIUM is a Antiviral that works by Acyclovir is a synthetic nucleoside analogue with activity against herpes simplex virus (HSV) types 1 and 2, and varicella-zoster virus (VZV). It is converted to acyclovir monophosphate by viral thymidine kinase, then further phosphorylated to acyclovir triphosphate, which competitively inhibits viral DNA polymerase and incorporates into viral DNA, causing chain termination.. OFIRMEV is a Non-opioid Analgesic that works by OFIRMEV (acetaminophen) is a para-aminophenol derivative with analgesic and antipyretic activity. Its mechanism of action is not fully understood, but it is thought to involve inhibition of cyclooxygenase (COX) enzymes in the central nervous system, with minimal peripheral COX inhibition. It may also act on serotonergic pathways and cannabinoid receptors.. They differ in pharmacokinetic profiles, FDA-approved indications, and side effect profiles.
Potency comparisons between ACYCLOVIR SODIUM and OFIRMEV 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 ACYCLOVIR SODIUM is: Dosing is indication-specific. For herpes simplex encephalitis: 10 mg/kg IV every 8 hours for 10–14 days (adults and children ≥12 years) or 20 mg/kg IV every 8 hours (3 months–12 years). For severe genital herpes: 5 mg/kg IV every 8 hours for 5 days. For mucocutaneous HSV in immunocompromised: 5 mg/kg IV every 8 hours for 7–14 days. For varicella zoster in immunocompromised: 10 mg/kg IV every 8 hours for 7 days. For neonatal HSV: 20 mg/kg IV every 8 hours for 14–21 days (disseminated/CNS) or 14 days (skin/eyes/mouth).. The standard adult dose of OFIRMEV is: IV: 1000 mg every 6 hours or 650 mg every 4 hours; maximum single dose: 1000 mg; minimum dosing interval: 4 hours; maximum daily dose: 4000 mg.. 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 ACYCLOVIR SODIUM and OFIRMEV 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. ACYCLOVIR SODIUM is classified as Category A/B. Pregnancy Category B. No evidence of teratogenicity in humans; fetal risks not established in first trimester. Use during pregnancy only if clearly needed.. OFIRMEV is classified as Category C. Acetaminophen (OFIRMEV) is generally considered low risk across all trimesters. No increased risk of major congenital anomalies has been consistently demonstrated. Chronic high-dos. Always consult a maternal-fetal medicine specialist before taking either drug during pregnancy or lactation.