Obama Explains Increasing Medical Marijuana Crackdowns, Raids In 'Rolling Stone' Interview

A simple law was not needed.  Existing law explicitly gives the executive branch the power to change the scheduling of particular drugs without needing Congressional action. Obama can instruct the relevant agencies under him to take an honest look at the research and reschedule marijuana so it qualify as having legitimate medical uses. The Obama administration could easily and justifiably move marijuana to, say, schedule III, which happens to be the same schedule that synthetic THC is in, making medical marijuana legal under federal law.

So Obama is either lying or he's ignorant of the law.
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