Teenagers using cannabis are causing long-lasting damage to their developing brains, a Canadian study suggests.
It found the impact on thinking skills, memory and behaviour was worse than that of teenage drinking.
The researchers, from the University of Montreal, urged teenagers to delay their use of cannabis for as long as they felt able.
The study tracked and tested 3,800 adolescents over four years, starting from around the age of 13.
Drinking alcohol and taking drugs, such as cannabis, at a young age is known to cause problems with cognitive abilities such as learning, attention and decision-making as well as academic performance at school.
This study found these problems increased as cannabis use increased - and the effects were lasting, unlike those of alcohol.
The teenagers, from 31 different Canadian schools, gave details of their drug and drinking habits once a year.
Their brain skills were also tested every year in school using computer-based cognitive tests.
Although levels of cannabis use in the study were low compared with alcohol use, 28% of the teenagers still admitted to some kind of use.
This compared with 75% of the teenagers who said they drank alcohol at least occasionally.
Prof Patricia J Conrod, lead study author, from the department of psychiatry at Montreal, said she had expected alcohol to have had more of an impact on the teenagers' brains.
But, instead, the research detected greater increases in errors in cognitive tests on the teenagers using cannabis - while they were taking the drug and after they had stopped.
These effects were seen in working memory, reasoning and their ability to control their behaviour.
"Their brains are still developing but cannabis is interfering with that," Prof Conrod said, of teenagers.
"They should delay their use of cannabis as long as they can."
She also said the findings highlighted the importance of drug-prevention programmes.
Cannabis is thought to be the most widely used illegal drug in the UK.
It can be addictive and using it has been shown to increase the risk of developing psychotic illnesses, particularly in adolescents.
Giving up cannabis can lead to withdrawal symptoms, such as sleeping problems and mood swings, experts say.
The study is published in the American Journal of Psychiatry.