The idea of using bottle caps to try and re-establish a currency is a pretty solid idea. You just need something small and durable which can be found all over the place and has no real physical use (much like coins). If everyone agrees that from now on these caps will be considered legal tender, then you've got yourself an economy. Narcotics wouldn't work as a currency because they have other uses, such as getting people high. And there'd be a lot of demand for that in a post-apocalyptic wasteland. There would probably be people producing more and shipping them around, but the quantity of narcotics in circulation would constantly be varying and that'd cause mad inflation. It would work in an anarchic society, but not if you're trying to rebuild the state.