My son, who was 11 years old at the time was started on 50 mg. He went as high at 75mg and as low as 25mg over the next year. They added allopurinol at one point too. Last week his GI put him back at 50 mg a day after a year of adjusting. Basically, it takes a while to find the right dose for you. This will depend on your weight and metabolism. Our Dr. said if you are feeling side effects the dose is probably too high. Although there is an adjustment period at first when you might have some side effects. They will monitor your blood levels so you get a "therapeutic level". Not too high or too low. My son is now 5'9" and 126lbs.
Make sure to drink a full glass of water when taking it and keep hydrated. We were also told to avoid milk products 1 hour before. My son finds it easiest to take before bed.
It has been a wonderful medication for him. I hope you have the same experience.