Allopurinol

Allopurinol is used in the prevention of gout. It works by inhibiting xanthine oxidase

Initiating allopurinol prophylaxis - see indications below

allopurinol should not be started until 2 weeks after an acute attack has settled
initial dose of 100 mg od, with the dose titrated every few weeks to aim for a serum uric acid of < 300 µmol/l
NSAID or colchicine cover should be used when starting allopurinol

Indications for allopurinol*

recurrent attacks - the British Society for Rheumatology recommend 'In uncomplicated gout uric acid lowering drug therapy should be started if a second attack, or further attacks occur within 1 year'
tophi
renal disease
uric acid renal stones
prophylaxis if on cytotoxics or diuretics

*patients with Lesch-Nyhan syndrome often take allopurinol for life

Interactions

Azathioprine

metabolised to active compound 6-mercaptopurine
xanthine oxidase is responsible for the oxidation of 6-mercaptopurine to 6-thiouric acid
allopurinol can therefore lead to high levels of 6-mercaptopurine
a much reduced dose (e.g. 25%) must therefore be used if the combination cannot be avoided

Cyclophosphamide