Say this
"I care about you and I am worried." "I have seen [specific behavior] and it scares me." "I called a place that can help, and they can get you in tomorrow." "I will drive you."
Not this
"You need to stop." "Why can't you just quit?" "You are ruining everything." "If you loved us you would stop."
Timing
Sober, private, and not in the middle of a crisis. Morning conversations tend to go better than late-night ones.
When they get defensive
Do not argue about whether there is a problem. Return to what you observed and what you are offering. Then let it rest and try again another day.