I used the expression Falling from grace to render the image of a "fallen person", someone who starts drinking too much or starts taking drugs because they feel lost, like celebrities, who might be on top of the world and next day they "fall from grace", and start doing what I said before.
So, if we were talking about celebrities, we could use the expressions I'm looking for.
You might say - people do say - somebody "was into drugs".
To be into is a general way to say that someone likes or is involved in something, not only drugs: "He was into model airplanes/ cooking/ politics." I can't think of phrasal verb that applies to drugs alone.
Ten to fifteen years ago when my kids were teen-agers, I heard them and their friends use both "drugging" and "doing" as slang verbs for using drugs.
"She's drugging again."
"Jason started doing after he hooked up with Shannon."
Of course, using doing in this sense is very context dependent. I also have no knowledge as to the extent to which those verbs are used in those senses. I do still occasionally encounter "doing" to mean "using drugs".
"Using" is also occasionally used by itself, with context, to mean using drugs.