Just to clear things up, adderall is 4 salts, 2 of which are dextro and 2 of which are racemic. This means that adderall is (approximately, because the salts have different weights) 75% dextro and 25% levo. In theory, then, 20 mgs adderall should be equivalent to 15 mgs dexedrine, only with slightly more peripheral stimulation.
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NO, adderall is NOT half active, it is half pure dextro salts. The other half is racemic, (and half of that, a quarter of the whole, naturally, is dextro, giving a grand total of 3/4s dextro.)
The levo isomer, incidentally, is not inactive, but it is believed to be less fun/therapeutic.
I will personally perform oral sex on anyone who can easily and consistently distinguish between 20 mgs adderall and 15 mgs dexedrine administered identically.
Adderall is a SCAM designed to A: get a patent for Shire and B: get rid of that "drine" suffix that has nasty associations for many parents. It worked wonders. Back in my "ADD" days, Ritalin was prescribed like candy and dexedrine was given as a last resort. Now amphetamines are immensely popular.