I believe that they are rarely used in diet pills now, though they were in the past (black beauty's), and if they still are it would be prescription only. Dextro-amphetamine is one of the active ingredients in adderall, which is used usually for add, it also contains other amphetamines that act more on the peripheral nervous system I believe. Dextrostat and Dexedrine are like adderall but they only contain dextro-amophetamine. Desoxyn is a pill used to treat I believe both add and narcolepsy, and it is pharmaceutical methamphetamine. Ive never had a proper experience with meth, but from what I gather it is similar to regular amphetamine but longer lasting and more addicting, it is also I believe neuro toxic.
Extacy is usually just a street name for MDMA, though nowadays a lot of Extacy pills are combined with amphetamine, methamphetamine, other stimulants MDAI, and cathinones like mephedrone