Amphetamine is not one thing, there are so many variants. MDMA (MethoxyDiMethylAmphetamine) is an amphetamine. Mescaline is an amphetamine.
There are several hundred amphetamine-like substances
Amphetamine is still used in war situations in low doses by several countries (source: Rusmidler i Norge 2016, Folkehelseinstituttet)
Street speed often contains residue from production of other narcotics and other additives like caffeine, salt, sugar.
1/4 gram usually contains 50-150mg pure amphetamine. Medicinal doses are 5-20mg
Methamphetamine is more fat soluble, so it acts quicker. Some meth also metabolizes into amphetamine, result is that meth kicks in quicker and lasts longer.
Effect; lowered concentration. Lowered ability to multitask.
Khat contains amphetamine-like compounds; katinon, katin, norephedrine.