ADVERSE REACTIONS Data upon which the following estimates of incidence of adverse reactions are made are derived from experiences reported in the literature, unpublished clinical trials and voluntary reports since marketing of ZYLOPRIM (allopurinol) began. Past experience suggested that the most frequent event following the initiation of allopurinol treatment was an increase in acute attacks of gout (average 6% in early studies). An analysis of current usage suggests that the incidence of acute gouty attacks has diminished to less than 1%. The explanation for this decrease has not been determined but may be due in part to initiating therapy more gradually (see PRECAUTIONS and DOSAGE AND ADMINISTRATION). The most frequent adverse reaction to ZYLOPRIM is skin rash. Skin reactions can be severe and sometimes fatal. Therefore, treatment with ZYLOPRIM should be discontinued immediately if a rash develops (see WARNINGS). Some patients with the most severe reaction also had fever, chills, arthralgias, cholestatic jaundice, eosinophilia and mild leukocytosis or leukopenia. Among 55 patients with gout treated with ZYLOPRIM for 3 to 34 months (average greater than 1 year) and followed prospectively, Rundles observed that 3% of patients developed a type of drug reaction which was predominantly a pruritic maculopapular skin eruption, sometimes scaly or exfoliative. However, with current usage, skin reactions have been observed less frequently than 1%. The explanation for this decrease is not obvious. The incidence of skin rash may be increased in the presence of renal insufficiency. The frequency of skin rash among patients receiving ampicillin or amoxicillin concurrently with ZYLOPRIM has been reported to be increased (see PRECAUTIONS). Most Common Reactions* Probably Causally Related: Gastrointestinal: Diarrhea, nausea, alkaline phosphatase increase, SGOT/SGPT increase. Metabolic and Nutritional: Acute attacks of gout. Skin and Appendages: Rash, maculopapular rash. *Early clinical studies and incidence rates from early clinical experience with ZYLOPRIM suggested that these adverse reactions were found to occur at a rate of greater than 1%. The most frequent event observed was acute attacks of gout following the initiation of therapy. Analyses of current usage suggest that the incidence of these adverse reactions is now less than 1%. The explanation for this decrease has not been determined, but it may be due to following recommended usage (see ADVERSE REACTIONS introduction, INDICATIONS AND USAGE, PRECAUTIONS, and DOSAGE AND ADMINISTRATION). Incidence Less Than 1% Probably Causally Related: Body As a Whole: Ecchymosis, fever, headache. Cardiovascular: Necrotizing angiitis, vasculitis. Gastrointestinal: Hepatic necrosis, granulomatous hepatitis, hepatomegaly, hyperbilirubinemia, cholestatic jaundice, vomiting, intermittent abdominal pain, gastritis, dyspepsia. Hemic and Lymphatic: Thrombocytopenia, eosinophilia, leukocytosis, leukopenia. Musculoskeletal: Myopathy, arthralgias. Nervous: Peripheral neuropathy, neuritis, paresthesia, somnolence. Respiratory: Epistaxis. Skin and Appendages: Erythema multiforme exudativum (Stevens-Johnson syndrome), toxic epidermal necrolysis (Lyell's syndrome), hypersensitivity vasculitis, purpura, vesicular bullous dermatitis, exfoliative dermatitis, eczematoid dermatitis, pruritus, urticaria, alopecia, onycholysis, lichen planus. Special Senses: Taste loss/perversion. Urogenital: Renal failure, uremia (see PRECAUTIONS). Incidence Less Than 1% Causal Relationship Unknown: Body As a Whole: Malaise. Cardiovascular: Pericarditis, peripheral vascular disease, thrombophlebitis, bradycardia, vasodilation. Endocrine: Infertility (male), hypercalcemia, gynecomastia (male). Gastrointestinal: Hemorrhagic pancreatitis, gastrointestinal bleeding, stomatitis, salivary gland swelling, hyperlipidemia, tongue edema, anorexia. Hemic and Lymphatic: Aplastic anemia, agranulocytosis, eosinophilic fibrohistiocytic lesion of bone marrow, pancytopenia, prothrombin decrease, anemia, hemolytic anemia, reticulocytosis, lymphadenopathy, lymphocytosis. Musculoskeletal: Myalgia. Nervous: Optic neuritis, confusion, dizziness, vertigo, foot drop, decrease in libido, depression, amnesia, tinnitus, asthenia, insomnia. Respiratory: Bronchospasm, asthma, pharyngitis, rhinitis. Skin and Appendages: Furunculosis, facial edema, sweating, skin edema. Special Senses: Cataracts, macular retinitis, iritis, conjunctivitis, amblyopia. Urogenital: Nephritis, impotence, primary hematuria, albuminuria.