Well the active drug is still the same, so there won't be a big difference in the side effects, the difference is that the XR is a sustained release, so you take it once a day and it lasts all day.
http://www.adderallxr.com/about_adderallxr/index.asp
When you take the regular
Adderall it is an instant release, so you get the whole dose at once and it only lasts a few hours in your body system, the XR is slowly released for hours, so you are getting a regular amount of the drug for the whole day.
This sometimes requires a dosage adjustment, if you are taking say 25mgs of regular Adderall, you might need 50mgs of the XR because less is released in your body at once, so the doc has to play around a bit to convert the dose and get the most effective amount of medication into your system.
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My information is not guaranteed correct. I do not get them right all the time, but I do enjoy the hunt~