What are the side effects of taking allopurinol like zyloric to reduce uric acid?

Diarrhea, Nausea: Diarrhea; nausea. Severe: severe allergic reactions (rash; hives; itching; difficulty breathing; tightness in the chest; swelling of the mouth, face, lips, or tongue); blood in urine or painful urination; dark urine; fever, chills, or sore throat; irritation of the eyes; joint pain; loss of appetite.

Very few: Allopurinol has been around for a long time and is safe for most. People with kidney failurs are at higher risk of allergic reactions including rashes. There is a very rare hypersensitivity syndrome which can be fatal.