Nytepassion--an F&F forum member and also a recovering addict--always used the best metaphor.
About the actively using addict, she would say, "He's on the bus."
Visualize your addict sitting on a moving bus, looking not out the window but straight ahead. You are racing alongside the bus, alongside his window, frantically calling, waving, anything to get his attention.
But he doesn't look at you, doesn't see or hear you. He's on the bus.
That is someone in active addiction. Anvil's post reminded me of Nytepassion's metaphor.