What is the adverse effects of febuxostat and allopurinol and to which patients they are not given and the other drug is used
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febuxostat allopurinol adverse effects comparison
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Febuxostat vs. Allopurinol: Adverse Effects & Contraindications
Both are xanthine oxidase inhibitors (XOIs) used for urate-lowering therapy (ULT) in gout and hyperuricemia. Here is a detailed comparison.
ALLOPURINOL
Mechanism
Purine analogue that inhibits xanthine oxidase. Metabolized to the active metabolite oxypurinol (half-life 15-30 hr), which is renally cleared.
Adverse Effects
| Category | Effect |
|---|---|
| Very common | Gout flare on initiation (always co-prescribe colchicine/NSAID for 6+ months) |
| Common | Skin rash (pruritic, ~3%), nausea, vomiting, diarrhea, LFT elevation |
| Rare | Peripheral neuritis, necrotizing vasculitis, bone marrow suppression, aplastic anemia, hepatotoxicity, interstitial nephritis |
| Rare/serious | Cataracts (allopurinol binds lens) |
| Life-threatening | Allopurinol Hypersensitivity Syndrome (AHS) - characterized by Stevens-Johnson Syndrome (SJS), toxic epidermal necrolysis (TEN), eosinophilia, fever, hepatitis, and renal failure. Occurs typically 2-4 weeks after starting therapy. |
Risk Factors for AHS
- Higher starting doses
- Renal dysfunction
- Concomitant diuretics, amoxicillin, or ampicillin
- HLA-B*5801 genotype (very high prevalence in Han Chinese, Korean, Thai populations)
Contraindications / Should NOT Be Used
- HLA-B*5801 positive patients - FDA recommends screening Han Chinese, Korean, and Thai patients before starting; allopurinol should not be used if positive. Use febuxostat instead.
- Prior allopurinol hypersensitivity - absolute contraindication; switch to febuxostat.
- Concomitant azathioprine or 6-mercaptopurine (6-MP) - allopurinol blocks their metabolism, causing severe toxicity (dose must be reduced by 75%, or ideally avoid the combination).
- Concomitant theophylline - allopurinol raises theophylline levels.
- Severe renal impairment - oxypurinol accumulates; requires careful dose reduction. Initial dose in CKD should not exceed 50 mg/day.
Key Drug Interactions
Azathioprine, 6-MP, theophylline, cyclophosphamide, oral anticoagulants (warfarin), amoxicillin/ampicillin, thiazide diuretics, cyclosporine, probenecid.
FEBUXOSTAT
Mechanism
Highly selective, non-purine xanthine oxidase inhibitor. Unlike allopurinol, it is metabolized primarily by hepatic oxidation and glucuronidation (CYP450). Renal elimination plays only a minor role.
Adverse Effects
| Category | Effect |
|---|---|
| Very common | Gout flare on initiation (co-prescribe colchicine/NSAID) |
| Common | Skin rash (<2%), nausea, arthralgias, LFT elevation (liver enzyme elevations are a notable effect) |
| Rare | Cytopenias |
| Controversial | Cardiovascular events (MACE) - Early concerns arose after the CARES trial (2018) showed higher all-cause and cardiovascular mortality vs. allopurinol. However, a 2024 systematic review (Ghossan et al., PMID 38115182) concluded that the increased cardiovascular risk has not been borne out in the totality of RCT evidence. A 2023 meta-analysis (Xie et al., PMID 38098046) similarly found comparable safety to allopurinol. |
Contraindications / Should NOT Be Used
- Prior febuxostat hypersensitivity - absolute contraindication.
- Concomitant azathioprine or 6-MP - same risk as with allopurinol; febuxostat also inhibits their metabolism.
- Concomitant theophylline - raises theophylline levels.
- Established cardiovascular disease (CAD, heart failure, stroke) - regulatory agencies in some countries (FDA label) warn against use in patients with established cardiovascular disease given the CARES trial signal, though evidence remains debated. Allopurinol is preferred in such patients.
- Severe hepatic impairment - febuxostat is primarily hepatically metabolized, so severe liver disease is a concern.
When to Use One Over the Other
| Situation | Preferred Drug | Reason |
|---|---|---|
| HLA-B*5801 positive (Han Chinese, Korean, Thai) | Febuxostat | High risk of life-threatening AHS with allopurinol |
| Prior allopurinol hypersensitivity / AHS | Febuxostat | Allopurinol contraindicated |
| Severe/moderate CKD | Febuxostat | Not renally cleared; allopurinol active metabolite accumulates in renal failure |
| Established cardiovascular disease | Allopurinol | Febuxostat carries an FDA warning for patients with serious CV disease |
| On azathioprine or 6-MP | Neither (if possible) | Both XOIs dangerously increase azathioprine/6-MP toxicity |
| Mild-moderate renal impairment | Either (dose-adjust allopurinol) | Febuxostat preferred if dose adjustment complex |
| First-line, no contraindications | Allopurinol (generally) | More evidence, lower cost, well-established |
Sources:
- Katzung's Basic and Clinical Pharmacology, 16th Ed., p. 1029
- Rheumatology 2-Volume Set (Elsevier, 2022), p. 7155-7218
- Firestein & Kelley's Textbook of Rheumatology, Table: Dosing and Safety of ULT
- Recent evidence: Ghossan et al. 2024 (PMID 38115182) - systematic review on cardiovascular safety of febuxostat; Xie et al. 2023 (PMID 38098046) - meta-analysis comparing febuxostat vs. allopurinol doses.
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