I disagree, rape is rape, whether intended or not, whether under the use of substances or not, rape is always rape.
The only circumstance I think potentially voids this is if the person is forced to take a substance against their will and loses consciousness. Then both are victims and the person who forced the substance on then rapist is to be held accountable.
If you willingly take a substance than force yourself on someone, you are 100% at fault regardless of impaired judgmental skills or consciousness.
I say this as a habitual drug user who's heard every damn excuse in the book for people doing horrid things.