Some people were asking about dosages, and I'd just like to give my 2 cents:
The first time I ever took amphetamine, I took 60mg of adderall, which gave me intense euphoria. I had to step it down after that. A few days later, I only took 40mg, which still caused fairly intense euphoria.
For a while, 20mg was a comfortable dose for me... then after a while, I had to take it back up to 40mg. Needless to say, my dose kept rising.
At one point, I was taking about 180mg at once, but this was just painful really.
Now, (I've been using amphetamine for something like 2 years) I've managed to figure out how amphetamine tolerance works with my body. I take 30mg (of adderall XR), wait 30 minutes to an hour, then take another 30mg (before the first 30mg kicks in). Then, an hour after I start to feel it, I take another 30mg.
By doing this, I have a comfortable high that lasts for hours, and I minimize the negative body load (although it is still going to be present at any dose).
Amphetamine tolerance works very differently from other drugs such as opiates. Your brain is very clever, and subconsciously realizes a correlation between the feeling that amphetamine gives and the skeletal muscle damage, vascular damage, and neuronal damage that amphetamine causes. Your brain actually rewires itself to turn off neuronal pathways that are in charge of producing pleasure
Sometimes, your brain goes to great lengths to avoid the damage that amphetamine causes, and will begin to make strong negative associations between whatever it is you are currently doing while on amphetamine and feelings of dread/terror/dysphoria -- almost like the movie A Clockwork Orange (where Alex can no longer listen to Beethoven's 9th symphony without feeling like he's dying).
Some of you may have noticed that you reach a point of amphetamine tolerance where "taking more" simply does nothing other than increase the negative physical effects.
There is almost a certain "ceiling effect" - where your body simply can't handle any more amphetamine, and no more positive effects can be produced. I'd say that this "ceiling dose" is about 120mg-200mg - depending of course on the individual, and how healthy they are.
Strangely (and no amphetamine user would say this doesn't hold true), it takes less amphetamine to achieve desired effects the healthier you are (the more sleep you have had, the more nutritious you eat).
You can also reduce your amphetamine tolerance psychosomatically (but be warned, amphetamine tolerance can also be increased psychosomatically as well)