How do you explain to patients with gout that the drug to prevent it (allopurinol) can initially cause it to get worse?

Be open: No need to discuss the pharmacology details, just describe it's important to get the gouty episode under control before starting the allopurinol.

See below: What i tell patients is that Allopurinol can acutely precipitate a flare due to the rapid drop in the serum concentration of uric acid when it is first started. I like to prime the system so to speak with the use of colcrys, prednisone, or NSAID when starting Allopurinol and that seems to prevent it from occuring.

Allopurinol and gout: Allopurinol (a) is of use in prevention of gout only if the person taking it makes too much uric acid (ua). A reduces ua production. If the person with gout has kidneys that secrete too little ua, a is of no use. These patients need colchecine (c). Before you start any drug therapy for high ua levels, see a physician to get a 24 hour urine for ua, so you can get the drug therapy that works best.

Hyperuricemia: The mechanism of Allopurinol directs at the recycling pathway of the purine metabolism by inhbiting metabolism of purine to Xanthine and Hypoxanthine as a Xanthine oxidase inhibitor. In the case of acute gout attack, Allopurinol can decrease urine excretion of uric acid causing increase hyperuricemia, which may exacerbates gout.