Sixteen and 17-year-olds are more than three times more vulnerable to developing a cannabis addiction than adults, researchers have found.
However, they may not be at increased risk of other mental health problems related to the drug, the small study suggests.
The findings indicate that adolescents who used cannabis were no more likely to have higher levels of subclinical depression or anxiety than adults who use cannabis.
Additionally, they were not more vulnerable than adult users to the associations with psychotic-like symptoms.
Lead author Dr Will Lawn, UCL Clinical Psychopharmacology Unit and Institute of Psychiatry, Psychology and Neuroscience at King’s College London, said: “There is a lot of concern about how the developing teenage brain might be more vulnerable to the long-term effects of cannabis , but we did not find evidence to support this general claim.
“Cannabis addiction is a real issue that teenagers should be aware of, as they appear to be much more vulnerable to it than adults.
“On the other hand, the impact that cannabis use has during adolescence on cognitive performance or on depression and anxiety may be weaker than hypothesised.
“But we also replicated previous work that if someone becomes addicted to cannabis, that may increase the severity of subclinical mental health symptoms.
“Given adolescents are also at a greater risk of experiencing difficulties with mental health than adults, they should be proactively discouraged from regular cannabis use.”
The study involved 274 people, including 76 adolescents (aged 16 and 17) who used cannabis one to seven days per week, alongside similar numbers of adults aged 26-29.
Along with non-users they answered questions about their cannabis use over the last 12 weeks and responded to questionnaires commonly used to assess symptoms of mental ill health.
On average the cannabis users in the study used the drug four times per week.
The adolescent and adult users were also carefully matched on gender, ethnicity, and type and strength of cannabis.
Researchers found that adolescent cannabis users were three and a half times as likely to develop severe ‘cannabis use disorder’ (addiction) than adult users.
This finding was in line with previous evidence using different study designs.
Cannabis use disorder is defined by symptoms including cravings, cannabis use contributing to failures in school or work, heightened tolerance, withdrawal and interpersonal problems caused by or exacerbated by cannabis use.
Half of the teenage cannabis users studied had six or more cannabis use disorder symptoms, qualifying as severe cannabis use disorder.
Among people of any age, previous studies have found that roughly 9-22% of people who try the drug develop cannabis use disorder, and that risk is higher for people who tried it at a younger age.
Adolescents might be more vulnerable to cannabis addiction because of a number of different factors, the researchers suggest.
This includes increased disruption to relationships with parents and teachers, a hyper-plastic (malleable) brain and developing endocannabinoid system (the part of the nervous system that THC in cannabis acts upon), and an evolving sense of identity and shifting social life.
The study, published in the Journal of Psychopharmacology, found that neither adolescent nor adult cannabis users were more likely to develop depressive or anxiety symptoms than non-users.
A separate paper, published in Psychopharmacology, found cannabis users were no more likely to have impaired working memory or impulsivity.
However, cannabis users were more likely to have poor verbal memory – remembering things said to them.
This effect was the same in adults and adolescents.
Senior author Professor Val Curran, UCL Clinical Psychopharmacology Unit, UCL Psychology & Language Sciences, said: “Our findings suggest that schools should be teaching pupils more about the risk of addiction to cannabis, which has been neglected in drugs education.
“Becoming addicted to cannabis is a serious problem in itself, but it can also increase the likelihood of other mental health problems.
“Teenagers should therefore be informed of their greater risk of addiction.”