As people have said, It would be up to the company to stipulate their own rules. I doubt (unless they felt it was ruining the companies image or what not) that a company would bother specifically banning something that is legal for day to day use (same as with alcohol).
Its only going to be a problem when it starts affecting you ability to perform to a good standard at work (the same as with anything). I can't imagine many employers would be too happy to have any one turn up high off their ass... same as if you turned up drunk, hungover, really sick or whatever).
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