Allopurinol is a potent inhibitor of the enzyme xanthine oxidase used in the treatment of hyperuricemia and gout. Since it is well known that purines exert multiple effects on pain transmission, we hypothesized that the inhibition of xanthine oxidase by allopurinol could be a valid strategy to treat pain in humans. This study aimed to compare the analgesic efficacy of oral allopurinol versus placebo as an adjuvant therapy in patients displaying fibromyalgia.
This randomized, double-blinded, placebo-controlled study included 60 women with diagnosis of fibromyalgia. Patients were randomly assigned to receive either oral allopurinol 300 mg (n = 31) or placebo (n = 29) twice daily during 30 days. The patients were submitted to evaluation for pain sensitivity, anxiety, depression and functional status before treatment, 15 and 30 days thereafter.
Oral administration of allopurinol 300 mg twice daily was ineffective in improving pain scores measured by several tools up to 30 days of treatment (p > 0.05). Additionally, no significant effects of allopurinol over anxiety, depressive symptoms, and functional status of fibromyalgia patients were observed in the present study.
Although previous findings indicated that allopurinol could present intrinsic analgesic effects in both animals and humans, this study showed no benefit of the use of oral allopurinol as an adjuvant strategy during 30 days in women displaying fibromyalgia. However, considering previous promising results, new prospective studies are still valid to further investigate allopurinol and more selective purine derivatives in the management of pain syndromes.

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