Well, I'm a Vet, not a physician, but I do know a little about Gout. It's caused, as you likely know, by the crystalization of uric acid in the joint...usually the big toe. Very painful I have heard. Allopurinal has been used for decades as a treatment.
When I want to know the details about a drug I'm told to take I just Google it like, "Allopurinal side effects". You'll get tons of hits...some in non-tech lingo. Read a few and see if you can deal with whatever the side effects are.
I'm sure an MD will chime in here pretty soon, though...or even better,a Pharmacist.
Here's a page from a Google search that might help:
www.askapatient.com/viewrating.asp?drug=18659&name=ALLOPURINOL
Looks basically pretty harmless to me...