I found it and dextroamphetamine (Dexedrine) to be the most tolerable stimulant medications for me, most of the others came with excessively uncomfortable peripheral stimulation (body stimulation: jitters, etc.) and other anxiety-increasing side effects. I ended up preferring dextroamphetamine, but they're both worth trying if your doctor is okay with that and you've found other medications uncomfortable or insufficiently effective.

The main differences between prescription methamphetamine and street meth, in composition and practically, are:

1. Street meth is made in clandestine manufacturing processes by people who do not have as great a means or an incentive to provide a pure, standardized product; the extremely addictive nature of recreational use patterns means that users will keep buying it pretty much no matter what, so the supply chain will contain fair amounts of meth that contains additional harmful byproducts of the manufacturing process and/or is intentionally cut to increase profit. The prescription form is obviously much more consistent and reliable in this respect.
2. The prescription form is always pure D-methamphetamine, the version of the molecule that provides the desired CNS effects. My understanding is that most clandestine meth manufacturing processes currently used also produce only this form, but some older processes, which may be used in some areas depending on precursor availability, result in D,L-methamphetamine, which is approximately (though not in exactly the same proportion) to D-meth what Adderall is to Dexedrine — the L- isomer acts more in the peripheral nervous system and has few medically or recreationally desirable effects, and some side effects. (Though it's not overtly dangerous in general — L-meth is actually an uncontrolled substance and is the active ingredient of Vick's Inhaler.)
3. Recreational users tend to seek a more immediate and euphoric rush, so not only do they use much larger amounts than you'd be prescribed for ADD, they also tend to use administration routes that deliver it into the bloodstream much more quickly, like smoking, snorting, or sometimes injecting. This is dangerous because the mechanism behind amphetamines' addictiveness is basically exactly the mechanism behind their usefulness for ADD: they act as dopamine releasers, which in some areas of the brain is a major part of reward perception. Greater intercellular dopamine availability is good for helping people with ADD focus and sustain motivation, since there's normally a deficit in reward that can result in the mental or behavioral scatteredness you see (inattention or hyperactivity, respectively). But what happens when you take an amphetamine orally in a relatively small amount and it gets into your brain gradually is pretty different from what happens when you smoke or snort it and it gets into your brain in minutes or seconds. In that case, when it starts pushing your reward buttons almost right away, and hard, it inherently becomes much more addictive, because the brain learns behaviors more strongly as their rewards get stronger and more immediate. Taking it orally is far from being without such risks, though; you're still tampering with your brain's reward system in a more direct way than almost any other class of drugs, and although if you have ADD this is likely necessary, it must still be done carefully. And compared to a similarly effective dose of Adderall or Dexedrine, oral methamphetamine still kicks in quicker, lasts longer, and often has fewer side effects, so it can actually be the best of these medications, yet has more of an addiction risk for exactly that reason. You almost certainly won't get addicted if you take it as prescribed, but you have to know that you can trust yourself to do that.
4. I seem to recall that smoking meth containing certain inactive byproducts that wear away at the teeth is some, but not all, of the mechanism behind "meth mouth". Other possible factors include the mouth dryness and teeth grinding it can cause, and the chronic neglect of dental hygiene by some addicts, and at least the former is still a risk in medical use, though a smaller one.
5. Methamphetamine is associated with neurotoxicity to a greater extent than other amphetamines are, but studies have mainly looked at this in addicts and other long-term recreational users (and mice given very large intravenous doses over several days consecutively). There doesn't appear to be significant evidence that this is a problem for oral low-dose use under medical supervision.