Side-effects
Because allopurinol is not a uricosuric, it can be used in patients with poor kidney function. However, allopurinol has two important disadvantages.
First, its dosing is complex,. Second, some patients 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, worsened renal function, and, in some cases, allopurinol hypersensitivity syndrome. Allopurinol is one of the drugs commonly known to cause Stevens–Johnson syndrome (SJS) and toxic epidermal necrolysis (TENS), two life-threatening dermatological conditions. More common is a less-serious rash that leads to discontinuing this drug. Studies have found HLA-B*5801 allele as a genetic marker for severe cutaneous adverse reactions that include Steven Johnson Syndrome and toxic epidermal necrosis caused by allopurinol.
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.
It is suspected to cause congenital malformations when used during pregnancy, and should be avoided whenever possible by women trying to conceive.
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