Teens, that is minors under the age of 18, need their parents to know if they are taking non-prescribed medication / drugs or alcohol.
In the U.S. there are certain confidentiality and patient rights granted to teens that are age 16 and up, but most practitioners make all patients (at intake) sign a form permitting them to tell parents / spouse / etc. if the person is hurting themselves or in imminent danger (self-harm, suicidal, etc.).
So, this is generally (ethically) dealt with at intake. There are certain areas where therapists and physicians are required reporters. For example, if they suspect abuse. They are legally required to report this – as are teachers, social workers, etc. This is part of their training.
Finally, often teenagers that are using alcohol or drugs are “self-medicating” – and frequently they need an intervention of sorts (seeking the root cause of why they are using) – and notifying the parents and getting the teen into a program that specializes in teen substance abuse is the typical protocol.