About 2 per cent of patients taking allopurinol will develop an allergic rash with the drug which will usually subside on with¬drawal of the drug. Sometimes, the rash does not recur when the allopurinol is given at a slightly lower dose, although this lower dose may be less effective in controlling the hyperuricaemia.

Other side-effects, commonly referred to as a general hypersensitivity, are much more serious and may even be life-threatening. These often include fever, dermatitis (with loss of superficial skin), liver disease, and inflammation of the kidney with rapid deterioration of kidney function.

This hypersensi¬tivity reaction may come on suddenly and be unpredictable. It can occur with other drugs and will often subside more rapid¬ly if treated with prednisone.
Which patients are likely to develop this side-effect cannot be predicted in advance, although there is some suggestion (which is not universally accepted) that the risk is greater if the dose of allopurinol is high or if the patient has renal disease or is taking diuretics. One needs to remember that allopurinol's effective¬ness in the presence of renal disease is one of its advantages. Still, the risk is small and serious hypersensitivity reactions occur in less than one in 10 000 patients taking allopurinol. The risk is slightly increased if the patient is taking the antibiotic ampicillin at the same time.

As with other allergic states, desensitisation to the drug may be undertaken for minor allergic side-effects, such as an itchy rash, although it is rarely undertaken and rarely successful for major sensitisation which involves kidney or liver disorder, with a fever. The success of desensitisation is unpredictable and it may be hazardous if the underlying hypersensitivity reaction was serious or life-threatening.

This puts into perspective my earlier comment that the use of a drug or medication is a balance between the good it does and the harm it may do. In most cases, the good outweighs the harm, but some potential for harm is always present with any effective medication and this harm will vary with the drug and with the individual patient. Thus a gout patient with renal disease would respond less well to a uricosuric drug, making allopurinol the drug of choice with the dose adjusted to renal function, but there is a slightly increased risk of drug hypersensitivity. In a patient with a known history of allopurinol hypersensitivity, a uricosuric drug would usually be needed.

However, if that patient had previously had renal colic from a renal calculus, there would need to be great caution in the administration of a uricosuric drug: to reduce the risk of further uric acid crystal and renal calculus formation it should be given only if the patient could consistently maintain a urine volume of at least 2500 mL per 24 hours and if that urine could be kept slightly alkaline with sodium bicarbonate.

Getting Rid of Gout - B. Emmerson