Head-to-head clinical analysis & difference comparison: details on mechanism of action, dosing, half-life, interactions, and maternal-fetal safety.
ULORIC vs ZYLOPRIM
Clinician-reviewed, head-to-head comparison of mechanism, dosing, pharmacokinetics, and safety profiles.
Last clinically reviewed: July 2026 · OpiCalc Medical Review Team
ULORIC (febuxostat) is a xanthine oxidase inhibitor that reduces serum uric acid levels by inhibiting the enzyme xanthine oxidase, which catalyzes the conversion of hypoxanthine to xanthine and xanthine to uric acid.
Allopurinol is a xanthine oxidase inhibitor that reduces the production of uric acid by inhibiting the conversion of hypoxanthine to xanthine and xanthine to uric acid.
Chronic management of hyperuricemia in patients with gout,Off-label: Prevention of tumor lysis syndrome (not FDA-approved)
Gout: management of chronic, primary, or secondary gout,Hyperuricemia associated with chemotherapy: prevention of acute uric acid nephropathy in patients with leukemia, lymphoma, and solid tumor malignancies receiving chemotherapy,Recurrent calcium oxalate calculi: prevention in patients with hyperuricosuria
40 mg orally once daily; may increase to 80 mg once daily if serum uric acid not at target after 2 weeks.
100-300 mg orally once daily, maximum 800 mg/day.
Terminal elimination half-life is approximately 5-8 hours. This short half-life supports once-daily dosing for maintenance of therapeutic urate-lowering effect.
Allopurinol: 1-2 hours; oxypurinol: 18-30 hours (prolonged to 48-72 hours in renal impairment). Clinical context: oxypurinol half-life determines dosing interval; dose adjustment required for Cr Cl < 20 m L/min.
Primarily metabolized by UGT1A1, UGT1A3, UGT1A9, and CYP2C8; minor metabolism by CYP1A2, CYP2C9, and CYP2D6. Approximately 22% excreted unchanged in urine.
Allopurinol is metabolized primarily by aldehyde oxidase to its active metabolite, oxypurinol; both are excreted renally.
Renal excretion of unchanged drug accounts for approximately 40-45% of the dose. Biliary/fecal excretion eliminates about 50-55% of the dose, primarily as oxidative metabolites.
Renal: allopurinol ~10% unchanged, oxypurinol ~70% unchanged; total renal elimination ~76% (allopurinol + oxypurinol); fecal/biliary: minor (~12-20% as allopurinol, ~3-5% as oxypurinol).
Approximately 99% bound to plasma proteins, primarily albumin and alpha-1-acid glycoprotein.
Allopurinol: <1% bound; oxypurinol: ~17-20% bound (primarily to albumin).
Apparent volume of distribution is about 50 L (approximately 0.7 L/kg). This suggests distribution into total body water and some tissue binding.
Allopurinol: ~1.6 L/kg; oxypurinol: ~0.4-0.6 L/kg. Clinical meaning: allopurinol distributes widely into total body water, while oxypurinol has a smaller Vd consistent with limited tissue distribution.
Oral bioavailability is approximately 85% (range 60-100%). Tablets are well absorbed, with food having no significant effect on overall absorption.
Oral: allopurinol 67-90% (mean ~80%); oxypurinol formed via first-pass metabolism has an effective systemic exposure.
No dose adjustment required for mild to moderate renal impairment (GFR 30-89 m L/min). Not recommended for use in patients with severe renal impairment (GFR <30 m L/min) or end-stage renal disease on dialysis due to lack of efficacy data.
Cr Cl >60 m L/min: no adjustment; Cr Cl 30-60 m L/min: 200 mg daily; Cr Cl 10-30 m L/min: 100 mg daily; Cr Cl <10 m L/min: 100 mg every 2-3 days or 50 mg daily.
No dose adjustment required for mild to moderate hepatic impairment (Child-Pugh Class A or B). Not recommended in severe hepatic impairment (Child-Pugh Class C) due to lack of data.
No specific guidelines; use with caution in severe hepatic impairment.
Safety and efficacy not established in pediatric patients; no FDA-approved dosing.
6-10 years: 150 mg/day; 11-16 years: 300 mg/day; <6 years: 50 mg/day; all given orally once daily.
No specific dose adjustment required; clinical studies included patients aged 65 and older with no overall differences in safety or efficacy observed.
Start at lower dose (100 mg daily) due to reduced renal function; titrate to achieve serum urate target.
Increased risk of cardiovascular death compared to allopurinol in patients with gout and cardiovascular disease. Febuxostat should be used only in patients who have not responded adequately to allopurinol or have contraindications to allopurinol.
None
Cardiovascular events: Increased risk of cardiovascular death, especially in patients with pre-existing cardiovascular disease.,Gout flare: May increase frequency of gout flares during initiation; prophylaxis with NSAIDs or colchicine recommended.,Liver enzyme elevations: Monitor liver function tests; discontinue if persistent elevation or signs of liver injury.,Thyroid disorders: Can increase TSH levels; monitor thyroid function.,Renal impairment: Dose adjustment not required; limited data in severe renal impairment.,Drug interactions: Use with caution with azathioprine, mercaptopurine, or theophylline; increase risk of toxicity.
Allopurinol hypersensitivity syndrome (AHS) including Stevens-Johnson syndrome (SJS) and toxic epidermal necrolysis (TEN); increased risk in patients with HLA-B*5801 allele; renal impairment requires dose adjustment; use with caution in patients with liver dysfunction; may cause drowsiness or dizziness; discontinue at first sign of rash or other signs of hypersensitivity.
History of hypersensitivity to febuxostat,Concurrent use with azathioprine, mercaptopurine, or theophylline (absolute)
Absolute: known hypersensitivity to allopurinol or any component of the formulation. Relative: concomitant use with didanosine; severe renal impairment (Cr Cl <10 m L/min) unless used for prevention of uric acid nephropathy during chemotherapy.
No specific food interactions; however, high-purine foods (e.g., organ meats, anchovies, sardines, scallops, game meats, beer) may counteract efficacy by raising uric acid. Grapefruit juice has no known interaction with febuxostat. Avoid excessive alcohol, especially beer and spirits, as they increase urate levels.
High-purine foods (e.g., organ meats, anchovies, sardines, mussels, beer) should be avoided as they increase uric acid levels. No significant food-drug interactions besides alcohol.
Pregnancy Category C: In animal studies, febuxostat caused fetal toxicity (reduced fetal weights, increased incidence of fetal malformations) at doses equivalent to 2-4 times the human exposure. There are no adequate and well-controlled studies in pregnant women. Use during pregnancy only if potential benefit justifies potential risk to fetus. First trimester: limited data; second and third trimesters: theoretical risk of uric acid reduction impacting fetal growth due to role of uric acid in fetal development.
Allopurinol (Zyloprim) is a xanthine oxidase inhibitor. First trimester: Limited human data; animal studies show no evidence of teratogenicity at clinically relevant doses. Second and third trimesters: No known increased risk; use only if clearly needed. Overall FDA pregnancy category C.
Excretion in human milk unknown; M/P ratio not determined. Because many drugs are excreted in human milk and potential for serious adverse reactions in nursing infants, a decision should be made to discontinue nursing or discontinue drug, taking into account importance of drug to mother.
Allopurinol and its metabolite oxypurinol are excreted into human breast milk. Milk-to-plasma ratio approximately 0.9-1.4 for allopurinol and 0.5-0.9 for oxypurinol. No adverse effects reported in infants. Considered compatible with breastfeeding given very low infant dose (<2% of maternal weight-adjusted dose).
No specific dose adjustments recommended for pregnancy due to lack of pharmacokinetic data. Physiological changes in pregnancy (increased plasma volume, renal blood flow, and glomerular filtration rate) may reduce serum uric acid levels; however, no dose modification studies have been conducted. Use lowest effective dose if treatment necessary.
No specific dose adjustment required during pregnancy. However, pregnancy can increase renal clearance; monitor serum uric acid levels and adjust dose if necessary. Maintain lowest effective dose.
ULORIC (febuxostat) is a non-purine xanthine oxidase inhibitor indicated for chronic management of hyperuricemia in gout. Avoid use in patients with ischemic heart disease or heart failure due to increased cardiovascular events in the CARES trial. Do not use in asymptomatic hyperuricemia. Titrate from 40 mg to 80 mg if serum urate target not reached. SCr monitoring is not required, but caution with severe renal impairment (Cr Cl <30 m L/min) – no data. Avoid concomitant with azathioprine, 6-mercaptopurine, or theophylline due to XO inhibition. Coadministration with NSAIDs or colchicine is safe for flare prophylaxis. Check liver enzymes periodically as ALT elevations >3x ULN occurred in 2%.
Monitor serum uric acid levels monthly until goal is achieved; titrate every 2-4 weeks. Avoid use in acute gout flares; start after inflammation subsides. Check renal function and adjust dose accordingly (Cr Cl <30 m L/min: max 200 mg/day). Consider HLA-B*5801 screening in Han Chinese, Thai, or Korean patients to prevent severe hypersensitivity. Allopurinol hypersensitivity syndrome is rare but life-threatening; discontinue at first sign of rash. Concomitant azathioprine or 6-mercaptopurine requires dose reduction to 25-33% of original.
Take ULORIC once daily with or without food. Do not crush or chew tablets.,You may experience a gout flare when starting ULORIC; you will be prescribed medication (e.g., colchicine, NSAID) to prevent flares for at least 6 months.,Seek immediate medical attention if you develop chest pain, shortness of breath, rapid heartbeat, or sudden numbness/weakness – these may indicate a cardiovascular event.,Avoid drinking large amounts of alcohol or consuming high-purine foods (e.g., red meat, shellfish) as they can increase uric acid levels.,Tell your doctor if you have a history of heart disease, heart failure, stroke, or liver problems.,Report persistent nausea, right upper abdominal pain, dark urine, or yellowing of eyes/skin – signs of liver injury.
Take exactly as prescribed; do not miss doses.,Drink at least 8 glasses of water daily to prevent kidney stones.,Report rash, itching, or swelling immediately; may indicate severe allergic reaction.,Avoid alcohol, especially beer and liquor, which can increase uric acid.,Use with caution if you have kidney disease; your dose may need adjustment.,Do not start or stop other medications like diuretics without consulting your doctor.,This drug prevents gout attacks, so continue even if you feel well.
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 ULORIC vs ZYLOPRIM, answered by our medical review team.
ULORIC is a Xanthine Oxidase Inhibitor that works by ULORIC (febuxostat) is a xanthine oxidase inhibitor that reduces serum uric acid levels by inhibiting the enzyme xanthine oxidase, which catalyzes the conversion of hypoxanthine to xanthine and xanthine to uric acid.. ZYLOPRIM is a Xanthine Oxidase Inhibitor that works by Allopurinol is a xanthine oxidase inhibitor that reduces the production of uric acid by inhibiting the conversion of hypoxanthine to xanthine and xanthine to uric acid.. They differ in pharmacokinetic profiles, FDA-approved indications, and side effect profiles.
Potency comparisons between ULORIC and ZYLOPRIM depend on the specific clinical indication. These are both Xanthine Oxidase Inhibitor agents and are not directly interchangeable by dose. A physician or clinical pharmacist should guide any therapeutic switching decisions.
The standard adult dose of ULORIC is: 40 mg orally once daily; may increase to 80 mg once daily if serum uric acid not at target after 2 weeks.. The standard adult dose of ZYLOPRIM is: 100-300 mg orally once daily, maximum 800 mg/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 ULORIC and ZYLOPRIM 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. ULORIC is classified as Category C. Pregnancy Category C: In animal studies, febuxostat caused fetal toxicity (reduced fetal weights, increased incidence of fetal malformations) at doses equivalent to 2-4 times the h. ZYLOPRIM is classified as Category C. Allopurinol (Zyloprim) is a xanthine oxidase inhibitor. First trimester: Limited human data; animal studies show no evidence of teratogenicity at clinically relevant doses. Second . Always consult a maternal-fetal medicine specialist before taking either drug during pregnancy or lactation.