Cannabis is fat soluble, not water soluble like alcohol, so it takes time for it to be removed from the system - all fatty tissues absorb it and then it releases slowly. Can take several days for it to clear the system. Also, these days the varieties of cannabis with different THC content are enormous - so 'smoking a joint' can have very different effects, depending on what type it is. This from an industrial chemist - which is why I know about such things.
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