Just tell the doc that you are not sure that the methylphenidate gave you the most effectiveness and that you would like to try something different for comparison. Some docs actually jump right to the amphetamine meds becasue they can be more effective.
The issue comes when they do not know you and are not sure you are not just seeking drugs; or when they have dealt with lots of parents that have negative reactions to the posiblity of using amphetamines on thier kids. Like everyone, docs do things based on thier experiences.
I'm 19, and was recently re-diagnosed with adult ADHD mixed type. On Tuesday, I'm going to talk to my doc about getting some medication prescribed, but I'm not sure which angle to push...