How do different traditions define "intoxicant"?

Since the mapping of Novel Psychoactive Substances (NPS) began about ten years ago, around 500 new subsatances has been discovered. This is a somewhat different situation than what was the case 2500 years ago. Some NPS are not illegal, and even it they are defined as pshychoactive they're not intoxicating but more like caffein ("upper") or melatonin (for sleep) or valerian (calming). What defines - in the different traditions - whether taking an NPS is against the five precepts or not? Is it the intention with which you take the substance? Is it the effect? Neurobiological damage? Physical damage? Frequency? Does legal status come in to the picture? Is this something one should take directly with the teacher? Or something you can define according to your own conscience?