There's a lot of danger to it. My friend who'd give me them would regularly take two, at 120mg. He's extremely successful, has since stopped, but a doctor let him know he had some scarring on his heart from a minor heart attack.
Really amazing albeit dangerous drug. he took them since he was 8 or 9 and had no issue with stopping. It doesn't have remotely the same crash as straight adderall(dextroamphetamine). After over a year I still forget to take it. At my current dose it helps me through really rough ruts. When taking breaks, the withdrawal lasts about 1-2 days and then I'm back to "normal".
All this being said, I really wish these rise-n-grinders would cut this abuse out before it gets further restricted. It's been life changing for me in my normal therapeutic dose.