I have heard many times that the widely used antihistamines are not considered to be psychoactive drugs. Yet their impact on ones cognitive functions can be substantial. According to some sources, sedatives are also attributed being psychoactive, but not antihistamines. What is the difference?
Besides that, it is well known that antihistamines cause drowsiness and sleepiness which are artificially altered states of mind because they are 1) not naturally-occuring, 2) have different mechanics and flow than their natural counterparts.
from experience I know these effects can be very strong, up to the state when one sees dreams while being awake. I can't find any scientific papers on that, but I have found many reports on the web and among people I've talked to.