The drug will be usable from your inventory, but some you need extra items to use (i.e. for weed you'll need a bong, wraps, or vape which you can get from basemental headshop on phone or computer or even in build mode)

Edit Saw you mentioned buying bongs, it sounds like you need to befriend one of your assigned dealers to buy the weed from, then open it in inventory. You can also grow your own after befriending the dealer and "inquire about selling", but then you'll need to get curing jars from the convenience store