A number of medications are useful for preventing further episodes of gout, including xanthine oxidase inhibitor (including allopurinol and febuxostat), and uricosurics (including probenecid and sulfinpyrazone). They are not usually given until one to two weeks after an acute attack has resolved due to concerns that it may worsen attack symptoms. These medications are often used in combination with either an NSAID or colchicine for the first 3–6 months. This treatment is not recommended until a person has suffered two gout attacks, unless destructive joint changes, tophi, or urate nephropathy exist. This type of treatment is delayed until this point because is not cost effective to offer this treatment sooner. Urate-lowering measures should be increased until serum uric acid levels are below 300–360 µmol/L (5.0-6.0 mg/dL). This treatment should be continued forever. These medications should be continued even if the person experiences a gout attack while on the medication.