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The medical marijuana dispensary -- The Farmacy -- is almost as famous as Muscle Beach, and gets a lot more traffic.  Tons of celebs are among the clientele.

A source close to James tells us ... James and his buddy had never been inside a marijuana shop, so they cruised in.  We're told James doesn't have a medical marijuana card, so weed was off-limits, but he spent time perusing the holistic aisles and left without buying anything.

He should check out that methadone clinic in Bev Hills.  It's fabulous!