Allopurinol Side Effects, Upsides, Downsides, Tips

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Allopurinol Side Effects. Severity: Major. You should check with your doctor immediately if any of these side effects occur when taking allopurinol:

  • Ankle, knee, or great toe joint pain
  • joint stiffness or swelling
  • rash
  • rash with flat lesions or small raised lesions on the skin
  • abdominal or stomach pain
  • agitation
  • ammonia-like breath odor
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