Cardiovascular disease
There is a correlation between uric acid levels and cardiovascular disease and mortality, and so allopurinol has been explored as a potential treatment to reduce risk of cardiac disease. However, the data is inconsistent and conflicting, and the use of allopurinol for use in cardiovascular disease is controversial. Independently of its effects on uric acid, it may also have effects on oxidative stress and inflammation. Side effects
Because allopurinol is not a uricosuric, it can be used in people with poor kidney function. However, for people with impaired kidney function, allopurinol has two disadvantages. First, its dosing is complex. Second, some people are hypersensitive to the drug; therefore, its use requires careful monitoring.Allopurinol has rare but potentially fatal adverse effects involving the skin. The most serious adverse effect is a hypersensitivity syndrome consisting of fever, skin rash, eosinophilia, hepatitis, and worsened renal function, collectively referred to as DRESS syndrome. Allopurinol is one of the drugs commonly known to cause Stevens–Johnson syndrome and toxic epidermal necrolysis, two life-threatening dermatological conditions. More common is a less-serious rash that leads to discontinuing this drug.More rarely, allopurinol can also result in the depression of bone marrow elements, leading to cytopenias, as well as aplastic anemia. Moreover, allopurinol can also cause peripheral neuritis in some patients, although this is a rare side effect. Another side effect of allopurinol is interstitial nephritis.
Drug interactions
Drug interactions are extensive, and are as follows:
Azathioprine and 6-mercaptopurine: Azathioprine is metabolised to 6-mercaptopurine which in turn is inactivated by the action of xanthine oxidase - the target of allopurinol. Giving allopurinol with either of these drugs at their normal dose will lead to overdose of either drug;  only one-quarter of the usual dose of 6-mercaptopurine or azathioprine should be given;
Didanosine: plasma didanosine Cmax and AUC values were approximately doubled with concomitant allopurinol treatment; it should not be co-administered with allopurinol and if it must be, the dose of should be reduced and the person should be closely monitored.Allopurinol may also increase the activity or half-life of the following drugs, in order of seriousness and certainty of the interaction:
Ciclosporin
Coumarin anticoagulants, such as warfarin (reported rarely, but is serious when it occurs)
Vidarabine
Chlorpropamide
Phenytoin
Theophylline
Cyclophosphamide, doxorubicin, bleomycin, procarbazine, mechlorethamineCo-administration of the following drugs may make allopurinol less active or decrease its half-life:
Salicylates and medicines that increase the secretion of uric acid
furosemide (see more on diuretics below)Co-administration of the following drugs may cause hypersensitivity or skin rash:
Ampicillin and amoxicillin
Diuretics, in particular thiazides, especially in renal impairment
Angiotensin-converting-enzyme inhibitors (ACE inhibitors)

Pharmacology
A common misconception is that allopurinol is metabolized by its target, xanthine oxidase, but this action is principally carried out by aldehyde oxidase. The active metabolite of allopurinol is oxipurinol, which is also an inhibitor of xanthine oxidase. Allopurinol is almost completely metabolized to oxipurinol within two hours of oral administration, whereas oxipurinol is slowly excreted by the kidneys over 18–30 hours.