Create “out" plans together. Saying “my parents would disapprove," doesn't carry much weight when a young adult is offered alcohol or drugs. However, you can still talk about what types of things can give your child an “out" in high pressure situations.

My job does random drug testing and I can't afford to lose it.
Thanks but I have to wake up really early tomorrow because.
I'm good. I'm my friends' ride home tonight.
I'm really trying to cut back.
It's just not really my thing. I'm just not interested.