How does Allopurinol work?

Allopurinol => enzyme xanthine oxidase inhibitor

Therefore blocks the conversion of the purine metabolites (hypoxanthine and xanthine) into uric acid.

NOTE: Allopurinol is used to prevent gout attacks, not to treat them once they occur.

Indications for use:

1. Recurrent gouty attacks (>3 within 1 year)
2. Presentation of Gouty tophi
3. Renal insufficiency or recurrent nephrolithiasis with hyperuricemia
4. Tumour lysis syndrome

References

1. Pacher et. al. Therapeutic Effects of Xanthine Oxidase Inhibitors: Renaissance Half a Century after the Discovery of Allopurinol. Pharmacol Rev. 2006 Mar; 58(1): 87–114.doi:  10.1124/pr.58.1.6