Head-to-head clinical analysis & difference comparison: details on mechanism of action, dosing, half-life, interactions, and maternal-fetal safety.
DI-METREX vs ALLEGRA-D 24 HOUR ALLERGY AND CONGESTION
Clinician-reviewed, head-to-head comparison of mechanism, dosing, pharmacokinetics, and safety profiles.
Last clinically reviewed: July 2026 · OpiCalc Medical Review Team
Combination of diphenhydramine (H1-antagonist) and pseudoephedrine (alpha-1 agonist). Diphenhydramine blocks histamine at H1 receptors, reducing allergic symptoms; pseudoephedrine causes vasoconstriction via alpha-1 adrenergic receptors, relieving nasal congestion.
Fexofenadine is a selective peripheral H1-receptor antagonist that inhibits histamine release from mast cells. Pseudoephedrine is a sympathomimetic amine that directly stimulates alpha-adrenergic receptors in the respiratory tract mucosa, causing vasoconstriction and reducing nasal congestion. It also has weak beta-adrenergic activity.
Symptomatic relief of seasonal allergies,Upper respiratory tract allergies,Nasal congestion,Sinus congestion
Relief of symptoms of seasonal allergic rhinitis (sneezing, rhinorrhea, itchy nose/palate/throat, itchy/watery/red eyes),Relief of nasal congestion associated with seasonal allergic rhinitis,Relief of symptoms of perennial allergic rhinitis,Relief of nasal congestion associated with perennial allergic rhinitis
4 mg orally once daily, increased to a maximum of 8 mg once daily if needed.
1 tablet (fexofenadine 180 mg / pseudoephedrine 240 mg) orally every 24 hours.
The terminal elimination half-life is approximately 12 hours, requiring twice-daily dosing for steady-state concentrations.
Fexofenadine: terminal half-life 14.4 hours (range 11-17 h, ~4-fold longer than IV due to enterohepatic recirculation); pseudoephedrine: terminal half-life 4.3-8 hours (alkaline urine prolongs to 16 h).
Diphenhydramine: extensively metabolized via CYP2D6 to inactive metabolites; pseudoephedrine: partially metabolized in liver via N-demethylation to active metabolite (norpseudoephedrine) and excreted unchanged in urine.
Fexofenadine is minimally metabolized (≤5% of dose) by the liver, primarily via CYP3A4; other minor pathways involve CYP2D6 and CYP2C9. Pseudoephedrine is partially metabolized in the liver by N-demethylation (CYP2D6) and oxidative deamination.
Renal excretion accounts for approximately 70% of elimination as unchanged drug and metabolites; biliary/fecal excretion accounts for the remaining 30%.
Fexofenadine: ~95% excreted unchanged in feces (80%) and urine (11-12%); pseudoephedrine: ~70-90% excreted unchanged in urine (major route).
Approximately 85% bound to serum albumin.
Fexofenadine: 60-70% primarily to albumin and α1-acid glycoprotein; pseudoephedrine: negligible protein binding (<20%, mainly to albumin).
Vd is 0.8 L/kg, indicating distribution into total body water and some tissue binding.
Fexofenadine: 5.4-5.8 L/kg (extensive tissue distribution, ~30-40 times plasma volume); pseudoephedrine: 2.6-3.5 L/kg (distributes into body water, crosses blood-brain barrier).
Oral bioavailability is 90% due to minimal first-pass metabolism.
Fexofenadine: ~33-40% (oral, decreased by fruit juices); pseudoephedrine: ~85-100% (oral, minimally affected by food).
GFR 30-50 m L/min: 2 mg once daily. GFR <30 m L/min: not recommended.
GFR 30-49 m L/min: 1 tablet every 24 hours; GFR 15-29 m L/min: 1 tablet every 48 hours; GFR <15 m L/min: contraindicated or not recommended.
Child-Pugh A: no adjustment. Child-Pugh B: 2 mg once daily. Child-Pugh C: not recommended.
No dose adjustment required for mild to moderate hepatic impairment (Child-Pugh A or B). Not studied in severe impairment (Child-Pugh C); use with caution.
Not established; contraindicated in children under 12 years.
Not recommended for children under 12 years. For age >=12 years: same as adult dosing (1 tablet every 24 hours).
Start at 2 mg once daily; titrate cautiously due to increased risk of hypotension and cognitive effects.
Elderly patients may have reduced renal function; assess renal function prior to use. Initial dose may be adjusted based on renal function. Avoid use in patients with hypertension or cardiovascular disease due to pseudoephedrine.
Not applicable (no FDA boxed warning).
None
Do not use in patients with severe hypertension or coronary artery disease; caution in hyperthyroidism, diabetes, glaucoma, prostatic hypertrophy, and MAOI use; avoid exceeding recommended dose due to risk of serious cardiovascular events; may cause drowsiness or excitability in children.
Cardiovascular effects (hypertension, palpitations, tachycardia, arrhythmias) especially in patients with pre-existing cardiovascular disease; CNS stimulation (insomnia, nervousness, dizziness, anxiety); risk of ischemic colitis; urinary retention (especially in patients with prostatic hypertrophy); increased intraocular pressure in patients with narrow-angle glaucoma; severe hypertension or coronary artery disease; MAOI use or within 14 days of discontinuation; use in renal impairment requires caution; avoid use with alcohol or other CNS depressants; caution in patients with hyperthyroidism, diabetes mellitus, or angle-closure glaucoma; elderly patients may be more sensitive to side effects.
Hypersensitivity to diphenhydramine, pseudoephedrine, or any component; severe hypertension; severe coronary artery disease; concurrent MAOI therapy or within 14 days; narrow-angle glaucoma; urinary retention; during or within 2 weeks of MAOI use.
Concurrent use of or within 14 days after discontinuation of monoamine oxidase inhibitors (MAOIs); severe hypertension; severe coronary artery disease; narrow-angle glaucoma; urinary retention; hypersensitivity to any component
Avoid alcohol entirely. Folic acid supplementation is often prescribed to reduce side effects; do not take any other folate supplements without approval. Caffeine may slightly increase absorption, but no specific dietary restrictions. Maintain adequate hydration to help prevent kidney toxicity.
Fruit juices (apple, orange, grapefruit) significantly reduce fexofenadine absorption; take with water only. Avoid high-fat meals as they may affect pseudoephedrine absorption. No specific restrictions for pseudoephedrine, but avoid excessive caffeine (coffee, tea, cola) to reduce additive stimulant effects.
DI-METREX (metformin) is classified as FDA Pregnancy Category B. First trimester: No increased risk of major congenital anomalies observed in human studies; some studies suggest reduced risk of neural tube defects in women with PCOS. Second and third trimesters: Risk of neonatal hypoglycemia and macrosomia reduced compared to untreated diabetes; no evidence of teratogenicity. Overall, benefits of glycemic control outweigh potential risks.
FDA Pregnancy Category C. First trimester: No adequate studies, animal studies show potential risk. Second and third trimesters: Risk unknown; associated with increased risk of gastrointestinal atresia and gastroschisis with first trimester pseudoephedrine use. Avoid in preeclampsia due to vasoconstriction.
Metformin is excreted into breast milk in small amounts with an M/P ratio (milk-to-plasma ratio) of approximately 0.35. Infant exposure is estimated at 0.2-1% of maternal weight-adjusted dose. No adverse effects reported in breastfed infants; however, caution in premature infants or those with renal impairment.
Lactation Risk Category L3 (Moderately Safe). Fexofenadine excreted in breast milk in low amounts; M/P ratio not established. Pseudoephedrine excreted into breast milk with estimated relative infant dose 4.3% of maternal weight-adjusted dose. May reduce milk production and cause irritability in infants.
No routine dose adjustment recommended. However, as pregnancy progresses, renal function decreases and volume of distribution increases, which may reduce metformin clearance. Dose should be titrated to glycemic targets, up to a maximum of 2500 mg/day in divided doses. Monitor renal function and consider dose reduction if e GFR < 30 m L/min/1.73 m².
Pregnancy increases clearance of fexofenadine; however, no specific dose adjustment recommended. Dose of pseudoephedrine should be limited to lowest effective dose due to potential vasoconstriction. Avoid extended-release formulations in pregnancy if rapid delivery is anticipated.
DI-METREX (methotrexate) has a long half-life; monitor for cumulative toxicity. Administer folic acid supplementation to reduce gastrointestinal and hematologic side effects. Use with caution in patients with ascites or pleural effusions, as drug accumulation can occur. Premedication with NSAIDs increases methotrexate toxicity. Always check liver function tests and renal function before each dose.
ALLEGRA-D 24 HOUR ALLERGY AND CONGESTION contains fexofenadine 180 mg and pseudoephedrine 240 mg extended-release. Avoid in severe hypertension, coronary artery disease, narrow-angle glaucoma, urinary retention, and concurrent MAOI use or within 14 days. CNS stimulation possible; monitor for insomnia, nervousness, and dizziness. Not recommended in patients with impaired renal function (Cr Cl < 60 m L/min) due to fexofenadine accumulation. Do not crush or chew tablet.
Take methotrexate exactly as prescribed, usually once weekly, not daily. Serious harm can occur if taken daily.,Avoid alcohol completely to reduce liver damage risk.,Report any unusual bleeding, bruising, fever, mouth sores, or persistent cough immediately.,Do not take any other medications, including over-the-counter and herbal products, without first consulting your doctor.,Use effective contraception; methotrexate can cause severe birth defects.
Take one tablet daily with water; do not crush or chew.,Avoid taking with fruit juices (e.g., apple, orange, grapefruit) as they may decrease absorption.,Do not use with other products containing pseudoephedrine or antihistamines.,Stop and consult doctor if symptoms do not improve within 7 days or are accompanied by fever.,Avoid alcohol and sedatives as they may increase dizziness.,Discontinue if signs of hypertension or tachycardia occur.,Contraindicated within 14 days of stopping MAOIs.,Pregnant or nursing women should consult a physician before use.
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 DI-METREX vs ALLEGRA-D 24 HOUR ALLERGY AND CONGESTION, answered by our medical review team.
DI-METREX is a Antihistamine-Decongestant that works by Combination of diphenhydramine (H1-antagonist) and pseudoephedrine (alpha-1 agonist). Diphenhydramine blocks histamine at H1 receptors, reducing allergic symptoms; pseudoephedrine causes vasoconstriction via alpha-1 adrenergic receptors, relieving nasal congestion.. ALLEGRA-D 24 HOUR ALLERGY AND CONGESTION is a Antihistamine-Decongestant Combination that works by Fexofenadine is a selective peripheral H1-receptor antagonist that inhibits histamine release from mast cells. Pseudoephedrine is a sympathomimetic amine that directly stimulates alpha-adrenergic receptors in the respiratory tract mucosa, causing vasoconstriction and reducing nasal congestion. It also has weak beta-adrenergic activity.. They differ in pharmacokinetic profiles, FDA-approved indications, and side effect profiles.
Potency comparisons between DI-METREX and ALLEGRA-D 24 HOUR ALLERGY AND CONGESTION 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 DI-METREX is: 4 mg orally once daily, increased to a maximum of 8 mg once daily if needed.. The standard adult dose of ALLEGRA-D 24 HOUR ALLERGY AND CONGESTION is: 1 tablet (fexofenadine 180 mg / pseudoephedrine 240 mg) orally every 24 hours.. 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 DI-METREX and ALLEGRA-D 24 HOUR ALLERGY AND CONGESTION 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. DI-METREX is classified as Category C. DI-METREX (metformin) is classified as FDA Pregnancy Category B. First trimester: No increased risk of major congenital anomalies observed in human studies; some studies suggest re. ALLEGRA-D 24 HOUR ALLERGY AND CONGESTION is classified as Category C. FDA Pregnancy Category C. First trimester: No adequate studies, animal studies show potential risk. Second and third trimesters: Risk unknown; associated with increased risk of gas. Always consult a maternal-fetal medicine specialist before taking either drug during pregnancy or lactation.