This can be a very touchy situation because many people do not feel they have addiction or they feel they can quit at anytime. I think the best way to approach it is tell them that you care about them and were just getting a little worried and that you are there to help in any way possible. Do not be aggressive and if they start being defensive make sure you keep calm and keep say that you will be there to help if necessary. I think this is a good first step. Good luck:)
Approaching someone about their addiction is such an important step that I really think it's worth getting professional advice. There are trained interventionists that can help you, or you can just speak to a counselor or doctor. I'd also recommend reading about different intervention methods. I don't think a full-blown intervention is always necessary, but I think it helps to be informed about addiction and the way someone is likely to respond to your approach. There are many different ways to go about it and one will probably feel right to you based on your personality and the personality of the person you're approaching. And while I think professional advice can be really helpful, don't let anyone push you into something that doesn't feel like the right thing to do.
Also, I think MissThng's advice is great and you should definitely keep that in mind.
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