The pseudoephedrine contained in many

The pseudoephedrine contained in many "non-drowsy" drugs like these is really an amphetamine in disguise, differing from methamphetamine by only one oxygen atom, a proton and a symmetry operation. These differences are sufficient, however, for the stimulant effects to be hindered by the passage of pseudoephedrine molecules across the blood-brain membrane, plus orders of magnitude difference in its activity where it counts - the dopamine neurons.