That rule applies JUST within the same substance. And only generally, some exceptions exist for poor bioavailability due to absorption differences and particula

That rule applies JUST within the same substance. And only generally, some exceptions exist for poor bioavailability due to absorption differences and particular metabolic pathways. The idea is that the shorter it takes to get into the bloodstream/brain, the peak effects are stronger, and the downregulation of neuron receptors is more acute and lasts longer (therefore requiring redoses to activate them).