Thanks for the great question about what to do if you suspect your teen is using drugs! Sadly, drugs seem to be so readily available to teens these days, that many think it's perfectly normal and perfectly okay. I cannot tell you how many teens have looked me straight in the eye and told me that because weed is "natural" it is actually good for you. And they often believe there are no risks or downsides whatsoever.
If you suspect your teen is using drugs, the first thing to try to understand is why. In my experience working with teens (and adults, for that matter) drug or substance use of any kind is a form of escape 100% of the time. Strong words, I know. But I believe they are true.
So for me the next logical question is to figure out what they are "escaping from." It could be worry or sadness, grief or stress, pressures of school or life or competing at some sport or other activity, troubled relationships with a dating interest, friends, YOU (the parents), or even boredom. Once you understand this, the next step is to try to talk to them . . . that doesn't mean yell or threaten or berate or degrade them. It means to genuinely try to put yourself in their shoes and show that you understand where they are coming from. You were a teen once. Try to remember what it was like to feel so out of control over everything in your life.
One of the reasons they aren't telling you what's going on in their lives is that they have figured out it's not always safe to tell you stuff. They know you might judge them or reject them or punish them in some way, and they don't trust you. To try to get through to them, you have to let them know they can trust you. You are on their side. And you are not simply going to try to control them (they hate that as much as you do.)
It is okay to have consequences for them if you find drugs or drug paraphenalia. It is okay to have rules and enforce them. In fact, they need rules and guidelines and structure. They need some sense of predicatbility of what will happen. They just need these things administered with love and compassion and not with an "I gotcha" attitude.
It will be difficult for all of you; it will take some time to build this trust. And teens will often push back with everything they've got. But stand strong, always speak to them with love and not rejection. The connection with them is the key. Build or rebuild the relationship.
A long-winded answer for sure (anyone who knows me will not be surprised!). But I hope it is helpful. Tell me what you think.