Even drugs deemed to be 'soft' can have profound effects on young users, a GP has warned.
Dr Íde Delargy of the Irish College of General Practitioners (ICGP) urged people not to underestimate the influence of cannabis on teens who may have underlying psychological problems.
Dr Delargy (inset) is director of the ICGP's substance misuse programme, and said some adolescents may use drugs like cannabis and alcohol to manage an undiagnosed mental health issue.
"You have a chicken-and-egg scenario going on, where young people use these substances to self-medicate a mental health problem," she said.
"It's important to rule out any mental health issues when treating young drug users, as some of these softer drugs can make things worse.
"There are a lot of young people who experiment with drugs and get away with it without any short-term consequences, but there is a cohort of people who can be profoundly affected," she added. "We can't know who they are, and you won't know until you start using the drug.
"Every child is different and often there may be an underlying problem there."
Dr Delargy also expressed concern about legalising cannabis, saying research had shown that long-term use of the drug could lead to increased risk of depression, anxiety and other mental health issues.
She said young people with drug issues don't tend to present to GPs - and those who do are often brought by concerned family members.
"What tends to happen is the young person runs into an acute event and ends up in the A&E department, because of an accident, fall or public order issue," she added.
Dr Delargy advised parents to "be vigilant, and try to keep calm in what can be a very difficult situation".
"It's very important to keep the channels of communication open," she said.
She also recommends seeking professional advice and considering the help of local drug services.