If everything is illusory and only perceived, the narcotic and the perception it produces, be it faint or vibrant consciousness (or unconsciousness), are likewise illusory.
The physical universe and all things therein are the result of beings' collective karma, but "physicality" is no more substantial than mental activity (it is dependently originated). They are not mutually exclusive of each other, though matter depends on mind. This begs the question that if this is so, why does physical injury (or narcotics) alter the mind is very clear ways?
This is the perception of the ripening of karma.
Huseng has it right.
The physical world is percieved by the eyes through the medium of light or vision, the ears through the medium of sound, and so on.
The world of "mental activity" (there must be a better term for this than mental activity....but right now I can't think of one)...is percieved through the medium of "conciousness" in that organ of perception we call our brain or sometimes our "Self".
It's easy for people to see something like vision and our eyes as arising from Dependent Origination....because they have an apparent physical existance outside of our "Self"....but people feel that their "Self" is somehow different from those physical things.
When the understanding of "Self" as also arising from Dependent Origination is understood, we can also understand that our perception of "Self" is also as much of an illusion as is our vision.
Now, just like fog can dim our vision, and many noises can confuse our hearing....so can pre-conceptions, predjudice, and misunderstandings confuse the mind or our percieved "Self"...by obscuring the "mental activity" so we do not percieve it clearly.
"Substances" or drugs obscure this mental activity.
But so do mistaken assumptions, pre-conceptions, cultural and other preconceptions also obscure our clear view of this mental activity.
Therefore, we must practice, to clear all our senses...including conciousnrss and mental activity...so we can percieve clearly.