The idea of staging an intervention is, however, an appealing one. It can seem right because it feels right to come together and share openly our hurt, our frustrations and concern. To be honest is a good thing, right? Well, just because something feels right, doesn't mean it's going to be effective.
Coming together as a group may very well be a good idea, and sharing hurts and frustrations may be very helpful - I'd encourage you to do this, but without the person present. Use it as an opportunity to get on the same page about how you can respond effectively, and how to present a united, compassionate front.
Make sure no one is enabling, intentionally or not. By this, we mean doing anything - usually money - that protects the person from the negative consequences of their use. If they're spending too much on their drug of choice, but you're giving them money because they tell you they can't pay their rent, then you're actually paying for their drugs.
If you are talking to the person, then tell them about the impact of the behaviour changes, not the drug use. Tell them about how you've worried, and what negative consequences you see. Share your love and your worries.
And, of course, no one should put up with bad behaviour, or put themselves at risk - including you. But on the other hand, setting a minimum level of support that will ensure safety is the counterbalance to enabling. For instance, while as a family you might all undertake to not give them money, you might ensure they always have a bed and a hot meal, as long as you feel safe.
Because caring and boundaries can be done at the same time, and change takes the person themselves to feel the impact of their actions - not being forced to by others. It's an annoying and inconvenient fact of human nature, but one that is unavoidable and one that we just need to work with - especially when we see someone we love in pain.