In the US methamphetamine is sold under the brand name Desoxyn, only in 5 and 10mg tablets though. It's exactly the same stuff as your friendly meth cook making shit out of pseudoephedrine makes, just with a guarantee of dosage and quality.
Meth is a stimulant, it's not that far off amphetamine, so it's meant to be prescribed in cases where plain amphetamine doesn't work.
What fucks people up isn't taking amphetamines all the time, it's not sleeping. If you retain a normal human sleep schedule and eat enough nutritious food to fuel your amphetamine antics (both of which can be really hard to do), you shouldn't feel too bad. If you're doing nothing but popping adderall and drinking Pepsi all day you are almost guaranteed to feel like garbage the next day...