Doing drugs
Before You Begin
Pause and breathe. Even if this shouldn't be happening... accept that it is and embrace it with composure and calmness.
Calmness is the only way forward because without calm, there is no possibility for receptiveness and connection. Your child won't give to you any more than they see from you.
Never forget that a strong connection is fundamental, as there will be no correction in them if there is no connection with you.
The Core Problem
There could be a wide range of underlying issues such as peer pressure, excitement seeking, stress avoidance, and/or coping with emotional pain.
What Will Make It Worse
Reacting with anger, shaming and/or disappointment will do nothing to bring them in closer to you, or put them into a state where they are ready to understand the dangers and make a change.
What The Child Needs
1. To really understand what drugs are going to do to them: The child needs to understand the health and legal consequences of drug use. Help them to see the depth of practical reasons behind "drugs are bad". Use online resources to paint the picture of what is inevitably going to happen to them if they continue.
2. Professional intervention (if needed): In cooperation with your child, meet with a professional who can assess the situation and provide appropriate guidance and support.
3. To find their reason to stop: Everyone has something that they won't let down. It could be their sports team, a person that looks up to them, their parents, their job, a pet. Help the child use that as motivation to walk away from their drug use.
4. A plan to say "no": Offer guidance for how to say no to peer suggestion/pressure when offered drugs in the future. Role-play different scenarios to help the child figure out how to remove themselves from the situation in a way that they are comfortable with.
How To Have The Conversation With Your Child
Find a moment when you think your child would be receptive to talking.
Share that you have seen evidence they are doing drugs, and that you love them too much to let that continue.
Tell the child you will not react negatively in any way, you just want to know why they started, and if it's the same reason they are still doing it.
Listen closely, and brainstorm with them what can be done to give them what they are trying to get from drugs from a much healthier source. If they are just bored, then have them start indoor rock climbing. If they are lonely, then integrate them into a shared interest community that they feel a strong connection to. Etc.
The next portion of the conversation should be taking them through the points under "What The Child Needs". Every point doesn't need to be talked through to completion, but they should be discussed enough that the message is taken to heart by the child and they are convinced that change needs to happen.
This is obviously not going to be an overnight fix, but this initial conversation will chart the path for you and them to work on ending their drug use and creating a much healthy and more fulfilling life for them.