The drug meth is actually just methamphetamine - there's no "nasty crap" in it unless the person synthesizing it doesn't refine and purify it well, which is true of basically all chemistry.

A more apt comparison is a strong beer and wine - both technically different things, but both get you drink because of the alcohol ingredient in them. They're both alcoholic drinks.

Same with amphetamines - both are stimulants that have essentially the same effect. That's specifically why they both work in treating ADHD. The only significant difference is one is legal (with a prescription) and the other isn't, which of course means good luck getting it really pure and at the correct dosage. The only other difference is how fast acting / long lasting they are because of that tail molecule, but the part that has an actual effect on people is the amphetamine part.